Goals from O'Brien and Willmott in the second-half were enough for Newport as Oldham recorded back-to-back defeats.
Making two changes to the team from the weekend, Afolayan and Branger were re-introduced from the start with De la Paz keeping his place in the Latics goal.
And it would be Oldham who would start the brighter once more. Two early corners in the opening minutes on either side were eventually dealt with by the hosts.
Then on the ninth minute mark, Edmundson headed over after a long throw by Hamer wasn't dealt with by Newport at the first time of asking in the box.
A minute later and De la Paz was forced to make a superb low save at his near post to deny Dolan who connected well after a cross from the right was fired in.
Seconds later and Latics went up the other end. Afolayan finding space on the left unleashed a shot across goal that went agonisingly wide of County's post.
Both teams who have had FA Cup giantkillings this season continued to trade blows by creating half chances, the most notable being a block by Clarke yards from goal to stop Matt's shot on target.
Just after the half hour mark, Branger was presented with the ball in the area, turning inside onto his left his effort was saved well by the body of Day after some confident attacking play by the lads in blue and white.
Then on the 35th minute the Oldham Captain landed awkwardly onto his back after a free-kick by Newport was floated in and punched away by Zeus.
Receiving instant treamtent and a lengthy stoppage, Hunt was brought on after Clarke was stetchered off to a round of applause by all those in attendance at Rodney Parade.
With seven minutes added on both teams went in at the break goalless.
The Welsh side came out the more attacking and took the lead on 47 minutes through their Captain, O'Brien who fired a shot into the Oldham goal after the initial effort wasn't cleared whilst Edmundson was down in the box injured.
The other centre-half for Latics who has played in every minute of every game for Oldham this season so far, also forced the hand of Wild in terms of changes after getting a heavy knock.
As Michael Flynn's men dominated on the hour mark, it was Amond's fierce strike that was pushed away by De la Paz at the near post that prevented the second goal from going in.
Oldham's spirited reaction saw possession on both wings but no clear-cut chances created.
The next chance came with 15-minutes remaining. Afolayan heading wide from close range after great play by Latics, the on-loan striker getting hurt in the process.
At the other end the Exiles then had another close range shot from Dolan pushed away at the near post by Zeus to keep the game at one-nil.
Not for long though as Willmott's deflected effort ten minutes before time on the right got a slight deflection before squirming its way in at the near post.
Newport who were looking to keep their play-off push alive for the last game of the season at the weekend had their free-kick by Dolan blocked and then held by De la Paz in the final minute of normal time.
Whilst with it being Latics' last away game of the campaign, it would result in back-to-back defeats on two long journeys South.
Attendance: 4,642 (160 Latics fans)
Newport County: Day, Butler, Willmott, Dolan (90' Pipe), Amond (93' McKirdy), Matt (81' Azeez), Sheehan, O'Brien (C), Poole, Demetriou.
Oldham Athletic: De la Paz, Hamer, Edmundson (54' Sylla), Clarke (C) (41' Hunt), Iacovitti, Maouche, Missilou, Lang, Nepomuceno, Branger, Afolayan (80' Benteke).