Shane Long bundled Ireland into the lead from a Robbie Brady corner after half an hour in Dublin, but De Jong nodded in Jetro Willems' cross with five minutes to go to snatch a draw for the visitors.
Ireland's first of two friendlies ahead of Euro 2016 - where they face Belgium, Italy and Sweden - saw Brady's set-pieces cause plenty of problems for Netherlands, but O'Neill accepted it was disappointing to concede a late goal.
"A bit disappointed to concede at the end, but overall I was really pleased," O'Neill said, adding he will not trim his squad to a final 23 before Tuesday's friendly against Belarus.
"It'll be highly unlikely [to cut any players]. Everyone goes to Cork.
"It's good for us to get the game organised and I think it'll be nice for the people of Cork."
O'Neill confirmed that strikers Robbie Keane and Daryl Murphy will play against Belarus, and singled out Harry Arter and Brady for particular praise after their fine performances against the Dutch.
"He's had a tough year with injury, but I think we'll see the best to come from Harry," O'Neill said. "I'm hoping he will be around for a long time.
"[Brady] is a very fine footballer, he's important to us and he is growing in stature."
The Ireland manager also looked ahead to Euro 2016, with an opening game against Zlatan Ibrahimovic's Sweden to come for O'Neill's men.
"It is a very important game and it'd be nice to get off to a good start," the 64-year-old admitted.
"[Zlatan] has been a very fine player for a very long time. We need to be very careful.
"We have to go to those games thinking we can compete, otherwise we may just stay at home."