Aertssen was born in Veldhoven on August 7, 2004, and moved from Sparta's youth academy to Ajax's youth academy in 2019. On January 11, 2023, he made his debut for the first team during the Dutch Cup match FC Den Bosch - Ajax: 0-2. That was his only match for Ajax 1, but the right-footed player played 63 matches for Ajax U23 and scored once.
Marijn Beuker: "Thanks to his incredible dedication and calmness, Olivier has taken wonderful steps during his time at Ajax. He was one of the captains of Ajax U23 over the last seasons for a reason. The competition and defense within the first team are big, so we gladly help with this transfer to Zwolle, where Olivier can play more and grow further. We wish him the best of luck there."