What have you found by looking both words up in good dictionaries (online or physical)? Have you looked at a synonym site to see if they appear to mean the same? We expect learners to show that they have done plenty of research themselves before asking us.
What I have found suggests they're synonyms, with 'oversize' marked as 'mainly US' in the Cambridge Dictionary and 'less frequent than oversized' in the Oxford Dictionary.
BrE speakers, would you then mostly go for 'outsize/d', say for clothes that are deliberately cut out of proportion not just XXXXL, or is 'oversize/d' fairly common too?
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