mass effect bought on steam

Is it giving you an error message?

I bought Dead Space 2 on Steam and Origin automatically updated my game library, though (if I remember correctly) it wasn't an immediate update.

They're competitors.  I don't think you can use a steam key in Origin, and vice-versa.  You can make an Origin game load from Steam, but it's still using Origin.

Somebody else can correct me if I'm wrong.

Mass Effect can't be redeemed on Origin unless it was bought on origin or if you talk to a game adviser. http://help.ea.com/contact-us