League Two Oxford raced into a two-goal lead through Johnny Mullins and Scott Davies as they attempted to make it third time lucky in a tie which had been postponed twice due to the state of Charlton’s pitch.
But Addicks defender Michael Morrison pulled one back with a powerful header at the start of the second half.
And the Championship side secured a draw and avoided an ignominious exit in their first match since they were taken over by Belgian businessman Roland Duchatelet thanks to Kermorgant’s acrobatic leveller.
The playing surface passed an inspection but still resembled that of a lower-league ground on third-round weekend.
So perhaps it was no surprise that Oxford, still a Conference club four years ago, adapted better to the conditions early on.
Chris Wilder’s side were back at the scene of a 4-0 Capital One Cup thumping in August – their only away defeat all season – but a repeat never looked on the cards as they attempted to earn a trip to Huddersfield in round four.
They survived an early scare when Charlton captain Johnnie Jackson’s far-post header from Lawrie Wilson’s cross flew wide.
But the rash of postponements meant Charlton had not played since New Year’s Day and the rusty-looking hosts fell behind in the 13th minute when they failed to clear Davies’ corner.
The ball landed at the feet of Mullins on the penalty spot and his shot took a big deflection before rolling through the mud and past the wrong-footed Addicks goalkeeper Ben Hamer.
Kermorgant could have fired Charlton level from another Wilson cross but opted to turn the ball back across goal rather than shoot and the chance disappeared.
And the hosts promptly found themselves two behind in the 24th minute when a stray pass by Jordan Cousins in his own half was intercepted by Mullins.
He found United striker James Constable, whose through-ball was perfect for Davies to fire low across Hamer and into the corner of the net.
Charlton trooped off to boos from the home fans at half-time but rallied after the break and halved the deficit when Morrison headed in Jordan Cook’s corner at the near post.
Oxford could have had a third but for a fine save from Hamer, who dived low to his right to claw away Constable’s header.
Instead Charlton hauled themselves level when Danny Green’s cross was volleyed in on the turn by Kermorgant 10 minutes from time.
The Addicks could have stolen it in stoppage time but David Hunt cleared Callum Harriott’s goal-bound header as Oxford hung on for a deserved replay.