Pablo Sarabia and Matheus Cunha struck for the 2-0 win on Saturday.
O'Neil later said, "Pleased for the group. They've given a lot and played better than we had today and came away with nothing. it was a gritty performance. Southampton are an awkward side to play against. We had to be patient. I thought we were excellent in the second half.
"We spoke a lot about this week about everyone being responsible. We did a lot of work on out-of-possession stuff. Half-time came at a good time for us and we could tweak it. It was a big clean sheet and I'm pleased for Jose Sa. He's had to work hard since Sam Johnstone came in."
On Cunha, he said: "He's huge for us. He has such ability but he is one of those players who drifts and takes up positions. We work very hard on him on how it is to be a trusted team player when we don't have the ball. He's worked very hard on it after I spoke harshly to him after Brentford away. A big win for us and Matheus Cunha had a big say in it.
"Brentford was a big mark for us. We went there and we were miles below our level. We should have got something off Man City here and then we've gone three unbeaten. It's going to be a big fight for us but we know we can turn it around."