Naruto Shippuuden Episode 124

[Buffalobiian]

Maybe the Sharingan got stuck on like Kakashi so he covers that eye (obviously if it was Obito he would have had to gotten a replacement non-Sharingan eye from somewhere, this would be the uncovered eye). Though the explanation for Kakashi was he needed it since he wasn't of Uchia blood, but I imagine it at least takes some Chakara for them as well.

Just playing devil's advocate with the theory though, I'd be disappointed if they did this special feature to show how one of the people close to Kakashi died just to have him be back.

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.

[DarthEnderX]

Well, considering everyone in Akatsuki except for Itachi and Hidan is a completely inhuman monster so far(sharkman, I guy with mouths all over his body, a guy sewn together with a bunch of hearts, a heart in a can...)his mask could be hiding any one of a million different monstrous powers.

[Jessper]

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.