James Constable fired Oxford in front in the 24th minute, but Matt Lund headed an equaliser for the visitors five minutes after the break.
"The second-half response from my team was magnificent," he said.
"I was very pleased with the performance. The points will take care of themselves when you put in performances like that.
"I think we have been a little bit hindered in the previous two away games by a sending-off and then by a penalty.
"But midway through the first half we started showing the belief that I know we have in us, and we took that on into the second half."
Oxford boss Chris Wilder saw his team lose their place at the top of League Two, with Chesterfield replacing them at the summit.
Wilder said: "I'm not angry about it, it was an entertaining game of football, I'm just a touch disappointed that we didn't kill off the opposition.
"We've kept the unbeaten run going and we are still the highest scorers in the league, but these last two home games have been missed opportunities.
"Some things are not quite going for us at the top of the pitch but our forwards will come good, I'm sure of that."