But United's 3-2 win over Bury means the top two sides are seperated by just goals scored, with both having earned the same number of points and boasting the same goal difference.
Glovers boss Johnson, who has been linked with managerless Coventry, hopes the loss will come as a warning to his players that there is still a lot of hard work to go before anyone is promoted.
He said: "I gave the players a short, sharp rebuke because we didn't play our game.
"Oxford imposed their game on us, and that doesn't normally happen. We normally put our game on others.
"We are not used to losing, but we'll bounce back. We have a lot of character - this is a kick up the backside."