Ogum is a biggish scary monster (not a boss, though) that’s not easy to find because there’s not many of them. You can find Ogums in the following dungeons (adventures): Solt Beach, Olkepth Caverns, Mount Olkepth.
Where are the most Ogums?
The most Ogums are in Solt Beach, where you can find 4-5 Ogums fairly often, although there are moments when you’ll find less.
Counterintuitively, you won’t find the most Ogums in Mount Olkepth (the hardest dungeon), where usually you’ll find 2 or 3. It’s not impossible to find more than that at times, though.
In Olkepth Caverns you usually find only 1 or 2 Ogums so it’s definitely not the place to farm them.