The result has further cast doubt on O'Neil's job at Wolves.
He said afterwards: "There are some areas in the pitch where we are blessed with quality. Matheus Cunha is a Premier League player, undoubtedly. Nelson Semedo - Premier League player, undoubtedly. Mario Lemina. We are going through the same issues and I will work to fix this and then also in January we can look to do that as well.
"The first place I look is at me. The first place they need to look is at themselves. They have to prove it.
"They have to step out there and prove it. They are a great group and I will protect them but they get judged on those 90 minutes. Within the game there will be good stuff but it's not enough. We need to win football matches."
O'Neil also said: "It is a big blow for the group. Especially with how we fought to get back into the game and be the better side for the second half, We were at maximum and felt like we would go on and win the game. To suffer that goal so late, the nature of the goal is crazy, which we have said already.
"I can shield them from so much but long balls over the top and then players are taking it upon themselves to change positions from a corner, for some reason. The subs made a big impact and we fought back and we have nothing to show for it."
On Ipswich's winner, he said: "I was told that it might have been offside. We have had a couple. There are far more pressing matters in my mind than VAR and whether the guy was in the way. Too many mistakes from us again. These cannot happen at these levels. We have no chance if we can't deal with Liam Delap running through. We have no chance if we can't stand in the right place and head the ball. It is a more than disappointing way to lose. We take nothing from it."
Asked about the most pressing matter, O'Neil added, "The team. The ability of the team to cope at this level. I have worked at this level for, this is my third year now and I have never had such a struggle to help the group cope with being at this level. We go to Everton and they boot it up the pitch and we can't cope. We have to find answers within the room to find answers. That second-half performance was us at maximum and them not really threatening. But we still found a way to concede two."