Ancient Greek Oratio Obliqua

Hello linguaphiles! I have a quick question about oratio obliqua and tenses therein. My question is: in Ancient Greek indirect statement after verbs such as λέγω in historic tenses, the verb in the ὅτι clause can be put in the optative, but it keeps the same tense as it was in the direct speech.

My question is, if there is only a present, aorist and future optative what happens if the original verb is imperfect, perfect, pluperfect etc.? Does it come down to aspect? In which case why a future?

Thanks :)