There is no rule like "if there is ㅗ in the front then add 아요" 행복해요 > 행복해 + 요 > 행복하다 + 요 > 행복 + 하다 + 요. 행복 (noun) 행복하다 (adj) 해 is shorten form of 하다. There is no word like 행복아요.

행복 is a noun. (happiness) 행복하다 is the adjective verb (to be happy) As you can see, in Asian languages, there are many verbs and/or adjective-verbs derived from nouns whereas it's the opposite in western languages where nouns and/or adjectives are derived from verbs.