and yeeaaaah there's a few reasons but it isn't anywhere near canon or anything, the closest franchise """evidence""" for it is basically that the creators intentionally designed him to make you suspect he was a siren, which are always female/dfab?
god and like...so, selkies in Oisin's world have certain rules and powers. Oisin's magic that he's studied for years is illusion magic. Which ranges from 'here's a pretty image' to 'you are now in this scenario, smell and touch it cos you sure can' to 'you're completely fine, but your nerves think they're on fire lol'
like, he's a criminal but he doesn't want to hurt people...it's just business! But yeah, he loves his magic and is devoted to studying it. It helps him stay in touch with his selkie side cos sad story there
you're already a little punk babythe fae don't give a shit for your gender nonsense, hence Oisin