An unconvincing Germany performance still ended in victory as they warmed up ahead of their World Cup opener against Japan next week.
The Werder Bremen attacker replaced fellow debutant Youssoufa Moukoko at half-time and scored with 10 minutes to play in a lacklustre encounter at the Sultan Qaboos Sports Complex.
Even with a pick-and-mix side that blended experienced faces with fresh blood, head coach Hansi Flick is likely to have some concerns a week out from his side’s Qatar 2022 curtain raiser against Japan in Group E.
Despite a performance below the standards Die Nationalmannschaft are likely to set themselves, the former Bayern Munich boss will hope to have found clarity rather than confusion in their encounter with Branko Ivankovic’s hosts.
Having started in-form Borussia Dortmund teenager Moukoko in attack, Germany pushed hard early on, with Leroy Sane their most dangerous operator.
An apparent problem for RB Leipzig’s Lukas Klostermann – who only returned from a knee ligament injury this month – checked any momentum though, with Armel Bella-Kotchap drafted in just after the half-hour mark.
With nothing to show on the scoreboard come the interval, Fullkrug joined the fray amid a slate of sweeping changes as Flick looked to find a way through.
Germany looked as if they would be frustrated, by both their own slow tempo and the stubborn resistance Oman presented throughout, with the hosts determined to give their heavyweight foes a challenge.
As the clock crept towards the final stages however, the visitors struck, with Fullkrug released between the defence to fire into the bottom corner and give his country the narrow win heading into Qatar 2022.