If Opportunistic Dragon leaves the battlefield before its triggered ability resolves, your opponent keeps control of their Human or artifact, it doesn't lose any abilities, and it can still attack or block as normal. You don't gain control of it at all.

If the stolen permanent normally has any abilities that would trigger when Opportunistic Dragon dies, those abilities won't trigger.

If another player gains control of the permanent Opportunistic Dragon has kidnapped, that permanent doesn't regain its abilities and still can't attack or block.

Opportunistic Dragon's effect doesn't expire if another player gains control of it or if it loses all abilities. The effect only expires once it's no longer on the battlefield.