Are there any rules for choosing between おお and おう?


Am I correct assuming that both おお and おう are homophones in Japanese - both
being pronounced as long o? I suspect I am, since that's what I'm indirectly
reading in various tutorials. If so, are there any (non necessarily
exhaustive) rules to guess the correct spelling of the words with long o? I
speak several languages where there are no strict spelling rules, but still
there are some hints that help guess the right spelling most of the time. Is
there such a rule for おお vs おう in Japanese?




Etymology. おお comes from an earlier おほ or おを, while おう can come from any of
おう、あう、おふ、or あふ (and potentially えう、えふ、ゑう、ゑふ if it's now よう). This is due to
sound change - originally all of these were distinct pronunciations, but they
have since been reduced to a single sound ([o:]).

Typically you can guess that [o:] in Chinese loanwords will be spelled with おう
(since neither おほ nor おを ever occurred in any Chinese loanwords), and that in
native Japanese words it will be spelled with おお (since the combinations that
led to おう were fairly rare compared to おほ and おを, though they do occur).




Etymology. おお comes from an earlier おほ or おを, while おう can come from any of
おう、あう、おふ、or あふ (and potentially えう、えふ、ゑう、ゑふ if it's now よう). This is due to
sound change - originally all of these were distinct pronunciations, but they
have since been reduced to a single sound ([o:]).

Typically you can guess that [o:] in Chinese loanwords will be spelled with おう
(since neither おほ nor おを ever occurred in any Chinese loanwords), and that in
native Japanese words it will be spelled with おお (since the combinations that
led to おう were fairly rare compared to おほ and おを, though they do occur).

