Originally a compound of 御(o-, “honorific prefix”) + 父(toto, “father”) + 様(-sama, “honorific suffix”).
/ototosama/ → /otottsan/ → /otossan/
The /otossan/ → /otoosan/ progression was artificial; regular Japanese sound changes generally would not allow this shift. The /otoosan/ form first appears in the early Meiji period in educational materials mandated by the 文部省(Monbushō, “Ministry of Education”).
Synonyms:(only used to talk about one's own father)父(ちち, chichi), (more general and impersonal)父親(ちちおや, chichioya), (more formal)父上(ちちうえ, chichiue), (more informal, similar to old man)親父(おやじ, oyaji), (Kagoshima)おやっどん(oyaddon)