Morris, whose contract at Elland Road was cancelled in March after off-the-field problems, angered Leeds fans by sprinting back to the halfway line with his hand to his ear in an over-the-top celebration.
Oster, fined by Leeds earlier in the week for a fracas outside a Durham nightclub, began his penance with a stunning first goal for the club.
David Healy ghosted past a defender and played the ball out wide to Oster who cut in from the right and blasted the ball through a crowded goalmouth into the net with his left foot.
It was a rare moment of class in a game punctuated with strong tackles and dogged commitment from both sides, though Millwall keeper Andy Marshall made spellbinding second-half saves to deny Healy and Frazer Richardson.
Millwall claimed they should have had a 63rd minute penalty when skipper Darren Ward fell to the turf, and keeper Sullivan pulled off a finger-tip save to deny Morris.
Leeds manager Kevin Blackwell predicted a tough, physical encounter and his words were quickly fulfilled with lectures for Sean Gregan and Barry Hayles, followed soon afterwards by bookings for Hayles and Jermaine Wright.
Brian Deane and Danny Pugh missed clear chances for Leeds, Deane heading fractionally wide of the far post after only six minutes and Pugh having his goal-bound drive cleared off the line by Millwall skipper Ward and later poking a shot wide from eight yards.
Healy, Gregan, substitute Simon Walton, Marcus Phillips and Ward were booked in the second half, and Gregan will now miss Leeds' Boxing Day visit to Sunderland through suspension.
The 84th-minute arrival of Adrian Serioux gave Millwall the extra option of his phenomenal long throw and it paid dividends within 60 seconds.
Serioux launched the ball into the penalty area from the right and Oster and Gary Kelly combined to upend Ifill.
Morris was surprisingly given the responsibility and thumped his penalty high to Sullivan's left.