"...from Proto-Germanic *AIRO " - hmmm, then Finnish apparently IS this mystic Proto-Germanic language, as OAR is exactly AIRO in Finnish! AIRO also in Votic, Izhorian, Karelian, Chud', AIR in Vepssian, AER, AERU in Estonian, AIRAZ in Livic. This word is also retained in Balto-Slavic speaks: AIRIS in Latvian, ORAS in Lithuanian.
According to Andres Pääbo, AIRO derives from AJAA (Fin.), AJA(MA) (Est.) - to push, drive, run, ride, operate, steer; HAJT (Hung.) - to drive, stimulate; AJE (Est.) - a push, a drive, a stimulus; AJU (Est.) - a run; AJUR (Est.) - a teamster - probably comes from the idea of 'men who push,drive' but can have been the source of 'oar' as the 'thing that pushes'.