Quote Originally Posted by Buffalobiian
The right-hand side of Obito was "crushed", while he gave his left eye to Kakashi. Tobi has the hole on his right.

If we say Tobi is Obito, then the open slot houses the sharingan.
Oops! Thought the mask had the hole for his left eye. Should have checked out a video before replying! Oh well, that would cover why he only shows one eye.