After last week's tension against Ipswich, this was another nerve-shredder and another occasion when the Millers hauled themselves over the line.
Strange things happen at this time of the season and it certainly applied as Dan Barlaser, who put the visitors in front with an early own goal, proved the comeback by scoring major bottle to assume responsibility from the spot after coolly fire home the hosts winner after Chiedozie Ogbene had been felled by Luke McNally.
Earlier, Rarmani Edmonds-Green had levelled for the Millers on the stroke of half time with Paul Warne's side showing all the best qualities of his time in charge with an energetic and assertive in-character second half as they secured a precious victory to keep hold of second spot.
That said, it was a tad edgy at times with the Millers desperate to avoid the play-offs and go up via the conventional route.
The majority of the first half was a tough watch in keeping with some testing occasions in recent times at the NYS.
It was sullen and frustrating, but fortunately it had one huge redeeming feature.
It came at the perfect juncture, courtesy of Edmonds-Green, who ghosted it to head home Ben Wiles's cross after home pressure following a couple of corners.
It was the first moment of pinpoint precision from the hosts and the perfect time for the Huddersfield Town loanee to net his first goal at home for the Millers.
Against a well-drilled Oxford side who knew their jobs off the ball and posed one or two problems on the break and were also streetwise in their ability to stop the hosts from getting their rhythm - and wasting a fair bit of time - it carried significance.
Prior to that, it had been a case of job done for the U's, who lead thanks to Barlaser's early own goal in trying to cut out a fine low cross from Sam Long, who got away too easily from Jordi Osei-Tutu.
At the other end, Oxford's defence got to grips - quite literally sometimes - with Michael Smith. Chiedozie Ogbene was kept in check, with the visitors coping with the aerial stuff from the hosts, who were denied a strong claim for a penalty prior to the U's opener.
It came when Richard Wood went down in the box following an excellent delivery from Barlaser, with the Millers defender appearing to be pulled down.
Edmonds-Green's goal intervention placed the onus back on Oxford to stop running down the clock.
Rotherham, meanwhile, were energised and went close on the restart when Barlaser narrowly failed to make amends for his own goal when he lofted a shot narrowly over after probing work by Ollie Rathbone and Ogbene.
More authoritative and convincing, the hosts then went close when Wood just failed to get enough purchase to nob the ball over U's keeper Jack Stevens, while at the other end, ex-Millers and Barnsley winger Ryan Williams spurned a good chance on the break with Viktor Johansson in the right place to block.
Both sets of fans grew more agitated in a half which was becoming open and frenetic with a draw not much use to either side in their respective quests.
It was the Millers who would make the next decisive move when Ogbene was bundled over in the box and Barlaser completed his personal comeback with his nerveless penalty.
With their tails up, the hosts pushed forward and Smith spurned a chance to seal matters when his curler flew over.
At the other end, ex-Middlesbrough winger Marcus Browne saw his effort saved by Johansson as the visitors sought to take their fates to a final game.
Smith - in possibly his last ever home game for Rotherham - then steered an effort wide from Ogbene's cross as the Millers sought a relieving third.
A glorious chance for that to transpire came and went when Ogbene - excellent in the second period - sent Wiles clear, with the midfielder appearing to stumble slightly before firing a low shot which was blocked by Stevens.
Rotherham United: Johansson; Edmonds-Green, Wood, Ihiekwe; Barlaser; Harding (Mattock 80), Rathbone (Lindsay 90), Wiles, Osei-Tutu; Ogbene, Smith. Substitutes unused: Chapman, Kelly, Bola, Kayode, Odoffin.
Oxford United: Stevens; Long, McNally, Moore, Brown; Williams (Browne 70), Kane (McGuane 45), Brannagan; Sykes (Whyte 78), Taylor, Bodin. Substitutes unused: Eastwood, Forde, Holland, Seddon.
Attendance: 10,415 (1,192 Oxford United fans).