The pair, both 20, look set to be given a chance against Charlton as manager Chris Wilder looks to juggle a busy period of fixtures.
While Ruffels has regularly featured this season, it is a year since Marsh started for the U’s, when he scored against Southend United in the Johnstone’s Paint Trophy.
Wilder has been impressed with both in training.
He said: “We’ve got midweek games to fit in after a couple of postponements and especially at the top of the pitch, I’ve got to make sure I get the balance right.
“People like Tyrone Marsh come into the equation. There might be opportunities for him and Josh Ruffels.”
United will have to make one enforced change from Saturday’s goalless draw at Portsmouth, with new signing Nicky Wroe cup-tied, but Dave Kitson is available again after serving a two-game ban.
Charlton will be without striker Marvin Sordell (hamstring). He is likely to be replaced by Simon Church, who scored two goals against the U’s in the 4-0 Capital One Cup tie at The Valley in August.
Charlton Ath: from Alnwick, Solly, Morrison, Wood, Wiggins, Wilson, Cousins, Stephens, Jackson, Church, Kermorgant, Hamer, Evina, Dervite, Pritchard, Cook, Green, Ajdarevic.
Oxford Utd: from Clarke, Hunt, Raynes, Wright, Newey, Williams, Mullins, Ruffels, Rigg, Marsh, Smalley, Constable, Kitson, Rose, Davies, Long, Bevans, Henly.