The word is "oh", but I'm not completely sure if it's "oh4" or "oh8", I think "oh4". In any case, it was quite a common way of saying "difficult" in Penang, when I was young. For example: "very difficult to do" = "cin-nia oh-co" (with sandhied/non-citation tone for the "oh").
I'll try and find it in Douglas/Barclay over the weekend.
Of course, "difficult to learn" would then be "oh-oh8"

Sim.