Silva conceded the penalty for the Blades' injury-time winner.
O'Neil later said: "It's not a penalty. I spoke to the referee, and I told him why it wasn't a penalty. He says there's contact – Fabio's shinpad probably grazes (Jack) Baldock's calf.
"I've watched it loads of times and there's minimal contact and if we're giving fouls for that, there would have been a million fouls in that football match.
"The referee is so quick to want to give it, but it's never a clear penalty. He needs to wait, be calm and if you've made a big error VAR will tell you.
"I've given up on penalties. I'm honestly lost, I don't know where to go with it. I'm told to go in and have a sensible conversation, but he still watched it telling me it's a penalty, which tells me we're in a crazy place.
"I know exactly what's happened, I played long enough, and Baldock knows just to move his foot across and he doesn't even wait for contact, he starts to go down before, and he doesn't see that which leads me to believe we're in a bad spot."
On standing by Silva, he added: "Of course, we lose here together. That's not on Fabio, that's me, the players and staff, we win and lose together.
"Fabio had a part to play in the penalty which wasn't a penalty. He was very upset. Young lad, he has my full backing and support, and we move on and make sure we do better next week."
