Goals from Johnny Mullins and Scott Davies saw the underdogs race into a 2-0 lead in the third-round tie at The Valley a week ago.
They led until seven minutes from time, when Yann Kermorgant’s volley rescued the Championship side and set up a replay.
After hitting back to avoid defeat, the Addicks are favourites to go through and visit Huddersfield in the fourth round on Saturday.
But Melville insists United have what it takes to spring another surprise.
The first-team coach said: “A lot of people will think have we blown our chance because we were in such a good position, but we don’t think so.
“We will prepare the same way and the mentality of the lads, as we’ve shown all season, means they will be right up for it.
“If anything, we can believe in ourselves a little bit more after the first game. We know we can compete with these sides and hopefully we can show that again.”
United make one enforced change as Nicky Wroe, their match-winner in Saturday’s 1-0 success at Wycombe Wanderers, is ineligible.
The home side will also be without Dave Kitson, who is out with an ankle problem suffered in the first game.
Manager Chris Wilder shuffled his pack at The Valley, bringing in Tyrone Marsh, Josh Ruffels and Davies.
Melville admitted a couple of them were unlucky to drop to the bench at Adams Park.
He said: “We talk about the development squad and people on the fringes doing well day-in, day-out in training.
“They’ve come in and are at the forefront of our minds now.”
Charlton’s away record is poor and a 1-0 defeat at Middlesbrough on Saturday extended their winless run on the road to six games.
Rhoys Wiggins was sent off on Teesside and is suspended, while full back Chris Solly has a knee problem.
Ben Hamer, who played in goal last week, has an ankle injury, so Ben Alnwick is most likely to start, despite sitting out the weekend with a virus.
Oxford Utd: from Clarke, Hunt, Raynes, Wright, Newey, Davies, Mullins, Ruffels, Rigg, Marsh, Constable, Smalley, O’Dowda, Rose, Williams, Bevans, Long, Shama.
Charlton Ath: from Alnwick, Wilson, Morrison, Wood, Evina, Green, Stephens, Cousins, Harriott, Kermorgant, Church, Phillips, Dervite, Hughes, Jackson, Pigott, Pritchard, Cook.