| May 17, 2026 | Thijssen | 1–0 | Ondersteijn | Round 5: Thijssen, Freek - Ondersteijn, Niels | Philidor Defense: Exchange Variation | |
| May 16, 2026 | Ondersteijn | ½–½ | Grooten | Round 4: Ondersteijn, Niels - Grooten, Herman | English Opening: King's English Variation, Botvinnik System | |
| May 15, 2026 | Guillaumond | 1–0 | Ondersteijn | Round 1: Guillaumond, Matthieu - Ondersteijn, Niels | Pirc Defense: Classical Variation | |
| May 14, 2026 | Bosch | 1–0 | Ondersteijn | Finals tiebreak: Bosch, Jeroen - Ondersteijn, Niels | Pirc Defense | |
| May 14, 2026 | Ondersteijn | 0–1 | Bosch | Finale en 3/4e plaats: Ondersteijn, Niels - Bosch, Jeroen | English Opening: Agincourt Defense | |
| May 14, 2026 | Freeke | 0–1 | Ondersteijn | Semi Final: Freeke, Matthieu - Ondersteijn, Niels | Trompowsky Attack | |
| May 2, 2026 | Ondersteijn | 1–0 | Van t Hoff | Round 9: Ondersteijn, Niels - Van t Hoff, Tom | English Opening: Anglo-Indian Defense | |
| May 2, 2026 | Kshirsagar | 0–1 | Ondersteijn | Round 8: Kshirsagar, Pranav - Ondersteijn, Niels | Bogo-Indian Defense: Nimzowitsch Variation | |
| May 2, 2026 | Ondersteijn | 0–1 | Grochal | Round 7: Ondersteijn, Niels - Grochal, Joey | English Opening: Agincourt Defense | |
| May 2, 2026 | Ondersteijn | 1–0 | Chua | Round 6: Ondersteijn, Niels - Chua, Ethan | English Opening: Anglo-Indian Defense | |
| May 2, 2026 | Ondersteijn | 1–0 | Van Ravensteijn | Round 4: Ondersteijn, Niels - Van Ravensteijn, Antal | English Opening: Anglo-Indian Defense | |
| May 2, 2026 | Geboers | 0–1 | Ondersteijn | Round 3: Geboers, Jimmy - Ondersteijn, Niels | English Opening: Agincourt Defense | |
| May 2, 2026 | Ondersteijn | 1–0 | Pol | Round 2: Ondersteijn, Niels - Pol, Nico | English Opening: Anglo-Indian Defense | |
| May 2, 2026 | Balde | 0–1 | Ondersteijn | Round 1: Balde, Esko - Ondersteijn, Niels | Bogo-Indian Defense: Grünfeld Variation | |
| May 1, 2026 | Fridman | 1–0 | Ondersteijn | Round 5: Fridman, Daniel - Ondersteijn, Niels | Bogo-Indian Defense: Nimzowitsch Variation | |
| Apr 11, 2026 | Ondersteijn | 1–0 | Baselmans | Round 9: Ondersteijn, Niels - Baselmans, Sam | English Opening: Anglo-Indian Defense | |
| Apr 6, 2026 | Chittka | 0–1 | Ondersteijn | Grenke Chess Festival 2026 Open | Bogo-Indian Defense: Grünfeld Variation | |
| Apr 6, 2026 | Ondersteijn | 0–1 | Eugene | Grenke Chess Festival 2026 Open | English Opening: Agincourt Defense | |
| Apr 5, 2026 | Fuhrmann | 0–1 | Ondersteijn | Grenke Chess Festival 2026 Open | Indian Defense: Tartakower Attack | |
| Apr 5, 2026 | Ondersteijn | 1–0 | Kupriyanov | Grenke Chess Festival 2026 Open | English Opening: Symmetrical Variation, Rubinstein Variation | |
| Apr 4, 2026 | Eljanov | 1–0 | Ondersteijn | Grenke Chess Festival 2026 Open | Pirc Defense | |
| Apr 4, 2026 | Ondersteijn | 1–0 | Kaufeld | Grenke Chess Festival 2026 Open | Hungarian Opening | |
| Apr 3, 2026 | Le Goff | ½–½ | Ondersteijn | Grenke Chess Festival 2026 Open | Indian Defense: Knights Variation | |
| Apr 3, 2026 | Ondersteijn | 1–0 | Bruns | Grenke Chess Festival 2026 Open | Réti Opening: Anglo-Slav Variation, with g3 | |
| Mar 28, 2026 | Ondersteijn | 1–0 | NO | Round 6: Ondersteijn, Niels - NO | ? | |
| Mar 7, 2026 | Van Foreest | 1–0 | Ondersteijn | Round 8: Van Foreest, Lucas - Ondersteijn, Niels | Pirc Defense | |
| Feb 7, 2026 | Ondersteijn | ½–½ | Tondivar | Round 7: Ondersteijn, Niels - Tondivar, Babak | English Opening: Agincourt Defense | |
| Dec 26, 2025 | Grooten | 1–0 | Ondersteijn | Round 5: Grooten, Tommy - Ondersteijn, Niels | English Opening: Agincourt Defense, Catalan Defense Accepted | |
| Nov 22, 2025 | Ondersteijn | 1–0 | Tjiam | Round 4: Ondersteijn, Niels - Tjiam, Dharma | English Opening: Anglo-Indian Defense | |
| Nov 7, 2025 | Grutter | ½–½ | Ondersteijn | KNSB competitie 2025/2026 | Pirc Defense | |
| Oct 25, 2025 | Delgerdalai | ½–½ | Ondersteijn | Round 7: Delgerdalai, Bayarjavkhlan - Ondersteijn, Niels | Pirc Defense | |
| Oct 24, 2025 | Ondersteijn | 1–0 | Barak | Round 6: Ondersteijn, Niels - Barak, Gil | English Opening: Agincourt Defense | |
| Oct 22, 2025 | Ondersteijn | 0–1 | Wedberg | Round 4: Ondersteijn, Niels - Wedberg, Tom | English Opening: King's English Variation | |
| Oct 21, 2025 | Ahmad | ½–½ | Ondersteijn | Round 3: Ahmad, Khagan - Ondersteijn, Niels | Catalan Opening: Open Defense | |
| Sep 22, 2025 | Ondersteijn | 1–0 | Martens | KNSB competitie 2025/2026 | English Opening: Symmetrical Variation, Botvinnik System Reversed, with Nf3 | |
| May 15, 2025 | Froeyman | ½–½ | Ondersteijn | KNSB Meesterklasse 2024-2025 | Pirc Defense: Classical Variation, Quiet System | |
| Apr 21, 2025 | Ondersteijn | 0–1 | Maksimovic | Round 8: Ondersteijn, Niels - Maksimovic, Bojan | King's Indian Defense: Fianchetto Variation, Classical Variation | |
| Apr 20, 2025 | Ondersteijn | 1–0 | Ceres | Round 6: Ondersteijn, Niels - Ceres, Dragos | English Opening: King's English Variation | |
| Apr 20, 2025 | Rozen | 1–0 | Ondersteijn | Round 7: Rozen, Eytan - Ondersteijn, Niels | Queen's Pawn Game: Symmetrical Variation | |
| Apr 19, 2025 | Ondersteijn | 1–0 | Aswath | Round 4: Ondersteijn, Niels - Aswath, S | English Opening: Agincourt Defense | |
| Apr 5, 2025 | Ondersteijn | 1–0 | De Schampheleire | KNSB Meesterklasse 2024-2025 | English Opening: Agincourt Defense, Catalan Defense Accepted | |
| Mar 8, 2025 | Korneev | ½–½ | Ondersteijn | KNSB Meesterklasse 2024-2025 | Pirc Defense | |
| Feb 8, 2025 | Ondersteijn | ½–½ | Beerdsen | Round 6: Ondersteijn, Niels - Beerdsen, Thomas | King's Indian Defense: Fianchetto Variation, Classical Variation | |
| Dec 14, 2024 | Spriestersbach | ½–½ | Ondersteijn | KNSB Meesterklasse 2024-2025 | Pirc Defense: Austrian Attack | |
| Nov 23, 2024 | Ondersteijn | 1–0 | Lenaerts | KNSB Meesterklasse 2024-2025 | English Opening: Agincourt Defense, Neo-Catalan Declined | |
| Oct 25, 2024 | Ondersteijn | 0–1 | Olsen | 39th European Chess Club Cup | English Opening: Agincourt Defense | |
| Oct 24, 2024 | Ondersteijn | 0–1 | Christodoulou | 39th European Chess Club Cup | King's Indian Attack | |
| Oct 23, 2024 | Bryakin | 0–1 | Ondersteijn | 39th European Chess Club Cup | Nimzo-Indian Defense | |
| Oct 22, 2024 | Ondersteijn | 1–0 | Bus | 39th European Chess Club Cup | Benko Gambit Accepted: Modern Variation | |
| Oct 21, 2024 | Laurusas | ½–½ | Ondersteijn | 39th European Chess Club Cup | Nimzo-Indian Defense: Normal Variation, Ragozin Variation | |
| Oct 20, 2024 | Ondersteijn | 1–0 | Quinn | 39th European Chess Club Cup | English Opening: King's English Variation | |
| Oct 15, 2024 | Ondersteijn | 1–0 | Bosman | Round 2: Ondersteijn, Niels - Bosman, Michiel | English Opening: Agincourt Defense, Catalan Defense Accepted | |
| Sep 21, 2024 | Ondersteijn | 1–0 | Elgersma | Round 1: Ondersteijn, Niels - Elgersma, Onno | English Opening: Symmetrical Variation, Rubinstein Variation | |
| Nov 9, 2024 | Slagboom | 1–0 | Ondersteijn | Round 3: Slagboom, Leandro - Ondersteijn, Niels | Nimzo-Indian Defense: Sämisch Variation | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Van der Lende | Round 2: Ondersteijn, Niels - Van der Lende, Ilias | English Opening: King's English Variation | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Grooten | Round 2: Ondersteijn, Niels - Grooten, Herman | English Opening: Symmetrical Variation, Three Knights, Fianchetto Variation | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Admiraal | Round 3: Ondersteijn, Niels - Admiraal, Miguoel | English Opening: King's English Variation | |
| Sep 14, 2024 | Ondersteijn | 0–1 | Rademakers | Round 3: Ondersteijn, Niels - Rademakers, Evert | English Opening | |
| Sep 14, 2024 | Ondersteijn | ½–½ | Van Foreest | Round 3: Ondersteijn, Niels - Van Foreest, Lucas | English Opening: Agincourt Defense, Neo-Catalan Declined | |
| Sep 14, 2024 | Ondersteijn | 0–1 | Van Foreest | Round 2: Ondersteijn, Niels - Van Foreest, Lucas | English Opening: Agincourt Defense, Neo-Catalan Declined | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Rademakers | Round 4: Ondersteijn, Niels - Rademakers, Evert | English Opening | |
| Sep 14, 2024 | Ondersteijn | ½–½ | Timmermans | Round 7: Ondersteijn, Niels - Timmermans, Mark | English Opening: Agincourt Defense, Neo-Catalan Declined | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Smeets | Round 9: Ondersteijn, Niels - Smeets, Kobe | King's Indian Attack | |
| Sep 14, 2024 | Ondersteijn | 1–0 | Skrobic | Round 11: Ondersteijn, Niels - Skrobic, Dragan | Nimzo-Larsen Attack: Indian Variation | |
| Sep 14, 2024 | Roorda | 0–1 | Ondersteijn | Round 2: Roorda, Thijs - Ondersteijn, Niels | Indian Defense: Wade-Tartakower Defense | |
| Sep 14, 2024 | Eerens | 0–1 | Ondersteijn | Round 2: Eerens, Pim - Ondersteijn, Niels | Pirc Defense: Byrne Variation | |
| Sep 14, 2024 | Beerdsen | 0–1 | Ondersteijn | Round 2: Beerdsen, Thomas - Ondersteijn, Niels | Pirc Defense | |
| Sep 14, 2024 | Blom | 1–0 | Ondersteijn | Round 3: Blom, Vincent - Ondersteijn, Niels | English Opening: Great Snake Variation | |
| Sep 14, 2024 | Van Delft | 1–0 | Ondersteijn | Round 3: Van Delft, Merijn - Ondersteijn, Niels | Zukertort Opening: Pirc Invitation | |
| Sep 14, 2024 | Eerens | * | Ondersteijn | Round 3: Eerens, Pim - Ondersteijn, Niels | ? | |
| Sep 14, 2024 | Blom | 0–1 | Ondersteijn | Round 1: Blom, Vincent - Ondersteijn, Niels | English Opening: Great Snake Variation | |
| Sep 14, 2024 | Van Batenburg | 0–1 | Ondersteijn | Round 3: Van Batenburg, Mees - Ondersteijn, Niels | Pirc Defense | |
| Sep 14, 2024 | Bijlsma | 1–0 | Ondersteijn | Round 5: Bijlsma, Nick - Ondersteijn, Niels | Pirc Defense: Classical Variation, Schlechter Variation | |
| Sep 14, 2024 | Lai | 1–0 | Ondersteijn | Round 6: Lai, Hing Ting - Ondersteijn, Niels | King's Indian Defense: Fianchetto Variation, Uhlmann-Szabo System | |
| Sep 14, 2024 | Beerdsen | 1–0 | Ondersteijn | Round 8: Beerdsen, Thomas - Ondersteijn, Niels | Pirc Defense: Byrne Variation | |
| Sep 14, 2024 | Van Arkel | 0–1 | Ondersteijn | Round 10: Van Arkel, Rogier - Ondersteijn, Niels | Pirc Defense: Austrian Attack, Weiss Variation | |
| Oct 6, 2023 | De Jong | ½–½ | Ondersteijn | European Chess Club Cup 2023 | King's Indian Defense: Fianchetto Variation, Uhlmann-Szabo System | |
| Oct 4, 2023 | Samadov | * | Ondersteijn | European Chess Club Cup 2023 | Catalan Opening: Open Defense | |
| Oct 2, 2023 | Santos Latasa | 1–0 | Ondersteijn | European Chess Club Cup 2023 | Sicilian Defense: Accelerated Dragon, Maróczy Bind | |
| Oct 8, 2022 | Prohaszka | 1–0 | Ondersteijn | ECCC_22 | Indian Defense: Wade-Tartakower Defense | |
| Oct 6, 2022 | Ondersteijn | 0–1 | Postny | ECCC_22 | Queen's Indian Defense: Fianchetto Variation | |
| Oct 4, 2022 | Maze | 1–0 | Ondersteijn | ECCC_22 | Pirc Defense | |