With the two sides level at 1-1, the game turned in a two-minute spell as Agbonlahor cleared a Liam Ridgewell header off the line and then headed the winner at the other end.
O'Neill told Villa's website: "That was a massive point in the game. Liam got up for his header and from a distance it looked as if it was going in. Gabby saved it and a couple of minutes later he scored for us. He's having a great time for us at the moment and I'm delighted for him.
"The winning goal was terrific and now I've experienced this derby. I was concerned about the game all week and wanting to do well.
"We've won the game and got the three points on the board, but it's about more than that. There's always more at stake in a derby game and the Villa fans have gone home delighted."