He could do it.
osasi → osaisi
I believe the default interpretation of “could” in the english sentence should be the conditional instead of the past tense.
On tatoeba, some people pointed at the ambiguity, but got rejected. A lot of the translations use the past tense, while only a few use the conditional interpretation.
User “CK”, native american, administrator and one of the biggest contributors to tatoeba, as well as advocate for high quality and idiomatic sentences has explained why the conditional is meant: