Gamekeeper at Hogwarts during Molly Weasley’s era, probably the man who held the job before Hagrid. Molly seemed to be rather fond of him (GF31).
Ogg's name has interesting associations from both the Bible and British folklore. In the bible, Ogg, King of the Bashan, tried to deter the Israelites from conquering the promised land; Ogg has become a name for giant in the Hebrew language. Ogg may also be a reference to the legendary British giants Gog and Magog who guarded the entrance to the royal palace. Their names also have their source in the bible. (--Brewer's Dict. and Lexicon reader Tomer Navot).
Ogg presents a bit of an inconsistency, at least on the face of it. We know that Hagrid was allowed to stay on at Hogwarts after being expelled, taking the job of Gamekeeper. How, then, can there be another Gamekeeper when Molly was at Hogwarts in the 1960s?
When we consider the question a little more, we see that it's unlikely that Hagrid was more than an apprentice at the age of 13. Perhaps Ogg was the man Hagrid worked under. The fact that Ogg was apparently still acting as Gamekeeper in the 1960s would suggest that Hagrid remained as an assistant for twenty years, until he was well into his 30s, before finally taking the job outright when Ogg retired.