I´m German too and I have never ever heard of that word.
Then google it first. Obersoldat was a rank that existed.
_________
That said, I believeit was really just the second lowest rank or so.
The rank of Oberschütze and its specific unit type equivalents (Oberkanonier, Obergrenadier- from 1942, Oberpionier, Oberfahrer, Oberfunker etc.) was created to give recognition and rank promotion to those enlisted soldiers who had achieved or displayed an above-average aptitude and proficiency but would not, however, qualify for promotion to the Gefreiter rank. A consideration for promotion to the rank of Oberschütze could usually be achieved after six months to one year of military service. In the militaries of other nations, Oberschütze was considered the equivalent of a private first class.[2]
I've read about a german soldier who was denied his Promotion to Gefreiter, because of his marriage to a jewish woman and because of that only achieved the rank of Obersoldat; something he wasnt very happy about.