The loss heaps pressure on O'Neil and his position.
He said afterwards: "We gave ourselves a chance to take something from the game. You can see the lads are still 100 per cent committed. We created some good moments and then we let ourselves down by losing our marker from the corner, which wasn't actually a corner because it came off Aaron Wan-Bissaka last.
"We didn't find a way to turn it in our favour today, but some of the decisions went against us in a big game in big moments.
"There are a lot of things that need to be done. They are a great group. They understand the situation at the club and that it's difficult. But they are standing up and fighting. There will be moments where we are not showing the quality.
"I am really disappointed because it was a decent away performance. We deserved some points. I watched Leicester City beat West Ham few days ago and Leicester were absolutely getting pummelled with 30 shots. This game was nothing like that, but Leicester manage to win that game and we come out with nothing."