The Championship side have admitted there is a serious problem with the drainage system at The Valley, which has caused them to cancel the third round tie twice already.
Their league game against Barnsley was also called off at short notice yesterday despite Charlton using a dome all week to dry out the playing surface.
The club apologised for the latest postponement, but in a statement on their website this morning reiterated they were confident the cup tie would go ahead.
It read: "A pitch inspection will take place on Monday afternoon ahead of the cup game with Oxford, but the club is expecting the tie to go ahead as planned."
Charlton intend to allow the pitch to dry out in the open air today, before setting up the dome, which is pumped full of hot air, later this afternoon.