Korean irregulars are, sometimes, kind of regular, because of rules for conjugation of different patchims. However, I realized today I don't know how to conjugate "regular" irregulars with a ㅂ patchim. Specifically, why do 돕다 and 곱다 get 오, while most others get an 우? I read somewhere that it's because they end in consonant + 오, but that is obviously not true (흥미롭다 -> 흥미로워). One other theory of mine was that it's because the previous syllable (i.e. the one before the ㅂ one) has an 오, so I imagined that because there is no syllable before 돕, for example, it gets this by default... until the obvious realization that this rule would hold true for 고맙다, but it doesn't.
So, are there any rules to adding 오 instead of 우? Or should I just learn these verbs by heart (are there even any other besides 돕다 and 곱다)?