Jaydon Banel kept Derby's play-off hopes alive with the winner in a 1-0 victory that pushed Oxford closer to League One.
The home side dominated for large parts of a game both teams needed to win for contrasting reasons and came out on top thanks to Banel's fine individual goal.
Oxford offered little in the first half but improved in the second only to miss two big chances with Myles Peart-Harris and Yunus Konak failing to hit the target.
Derby also had opportunities to increase their lead but held on for a sixth consecutive victory at Pride Park that keeps them in the hunt for a top six finish.
Jacob Widell Zetterstrom was back in goal for Derby after missing 13 League games with a virus but the early action was in the Oxford half as the home side started strongly.
Banel had a shot charged down before Derry Murkin's effort was deflected behind for a corner that saw Oxford break quickly but Stanley Mills was denied by good defending from Lewis Travis.
That was a rare Oxford attack and they fell behind in the 22nd minute through Banel who won the ball in his own half and surged forward, holding off two defenders, before firing low into the bottom left corner from the edge of the box.
The rest of the half was one way traffic and Carlton Morris had a good chance to double the lead in the 44th minute but headed over from an unmarked position at the back post.
After such a tame showing, Oxford had to improve and they had a big chance in the 56th minute.
The ball broke to Peart-Harris who ran clear but Zetterstrom came out to make a good block and Will Lankshear could do nothing with the rebound.
That was a big moment and one Oxford could not afford to miss in their fight to stay in the Championship.
It was looking like one of those afternoons for the visitors who had another chance to level in the 69th minute when a mistake from Matt Clarke let Konak in but he sliced his shot well wide with substitute Jeon Jin-Woo in an unmarked position.
Derby could not find the second goal that would have settled the game and there was one more chance for Oxford in stoppage time.
Aidomo Emakhu got to the byline and whipped in a low cross but Mark Harris could not turn it home at the near post.
Derby's John Eustace:
"It's a brilliant win against a really difficult opponent who make it very hard for you to break them down and I thought we played some really good football.
"We could have been better in the final third but overall delighted with three points. When you look at the team they put out, they've had a lot of draws and I think only two defeats in 10 so they're difficult to play against."
"In those kind of games you need that bit of magic, that moment of quality and Jaydon certainly brought that.
"He's a big player for us now, he stood up today. I thought it was a great learning experience for him away at Southampton last week and he's come back with the winning goal today. His work rate was very good so I'm delighted there's been good progression there.
"I think what let us down a little bit today was the decision making. We got into some really good areas and the final pass wasn't good enough."
Oxford's Matt Bloomfield:
"I've got a mixture of emotions. Disappointment at our performance first half and frustration that we haven't walked away with something at the end of the game.
"We've created two or three really good opportunities against a really good team who are pushing for a play-off position so it was always going to be a tough game for us.
"But we've created some really good chances and I'm really frustrated that we haven't taken one of them.
"No team in our position is going to create loads and loads of chances so when they come around you've to take them.
"In recent games we've been taking them but today, disappointingly, we didn't take one and we've had some real clear-cut chances.
"In terms of the reaction in the second half I thought fair play to the boys, they left everything out there and we took a very good team to the wire.
"It leaves us in a position where we still have games to win and points to put on the board. Today wasn't safety or relegation, we have to recover and we need to be better in the first half on Tuesday than we were today."
