You can watch OSL, OSC and all Proleague matches live. MSL is excluded because GOM TV has exclusive rights for that. (Just like OSL is not watchable on GOM)
Here's how to install:
Update: Download here
Go to http://tvpot.daum.net/live/
Just click on one of the rooms and after 1 or 2 commercials you should see the live stream.
You can also watch VODs for free, but unfortunately you need to sign up for that and thus need a Korean passport ID.