You can receive all erc20 tokens just by using a ethereum address of your wallet. To watch tokens in your wallet, you need also contract address of the tokens you receive
How does one get the contract address for OMG tokens received? Lets say if I send them from my Bittrex account?
Google is simplest way.
Just write down OMG smart contract, open etherscan link and you will find smart contract address, you will find decimals if you click on "read smart contract". In MEW click on "add token" and fill up blanked fields with it.