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:父(ちち, chichi)(only used to talk about one's own father), 父親(ちちおや, chichioya)(more general and impersonal), 父上(ちちうえ, chichiue)(more formal), 親父(おやじ, oyaji)(more informal, similar to old man), おやっどん(oyaddon)(Kagoshima)