The 31-year-old has scored one goal in 15 appearances for the Brewers this season but has not been handed a start since October.
Richards had netted 11 times last season following his move from Port Vale in the summer.
Oxford boss Chris Wilder told his club's official website: "Justin is a strong lad who scores his share of goals and also holds the ball up well and brings other people into the game.
"We needed an experienced player up front and he fits the bill very well."
The former Cheltenham forward will be ineligible when Burton travel to the Kassam Stadium on Tuesday January 29.
- Sports & Recreation