Actually, OBK is english historically correct short.
OKW is proper in polish.
What? No. English has taken the original abbreviation. [edit] Ober is one word, why would they put two letters from one word and then omit the W at the end to boot? No.
Anyway, even if it were "proper English", in the linguistic context we are all using (CoH2) it's called OKW. So using another acronym would be wrong. OKW is also correct in German, which is the language spoken by the game's faction. I don't se why Polish is at all relevant here
Do you have a source that says OBK is correct? Every source I've found says that OKW is correct and that OBK is a Spanish techno group.