I think human eyes are full grown at birth... So Oro could have taken Sasukes Eye at anytime. I think he doesn't just want the eye because then he might not be able to turn it off like Kakashi, also Sasuke only has 2 dots in it so he might want him to mature it a little first. My guess he wants either to have Sasuke defeat itachi or to somehow steal Sasukes body so he himself will get 2 fully functioning Sharigans.