Starting EA Origin in Offline Mode

From this forum thread, the parameter is "Origin.exe /StartOffline".

Playing Sims 3 on Origin Offline

I'll keep this brief.  I lost my internet connectivity all morning and thought it was a good time to test Origin and Sims 3.  All that I've seen online suggests that Sims 3 will only work online due to Origin's insistence of logging in when running the game.  So when I ran Origin, it gave me a login window and a note that Origin was not available (due to the internet).  I logged in anyway, and then Origin went to my library but also setting the client to Offline mode.  I was able to launch Sims 3 and Mass Effect 2 like this without issues.

A small note on Mass Effect... you will not be able to connect to the Cerberus Network.  You will get an error message but you'll be able to load your save just fine.