The game should work with Yosemite. A couple people have reported issues with the game after upgrading from Mavericks, though.
Work on officially supporting Yosemite started on or before the gold master release, and those involved were using the betas before then.
One person reported an issue recently creating an account with Chrome or Safari. If that is the case for you, try a different browser, if available, or email me (raze@larian.com, or the support address below) with your desired user name and I can set up an account for you with a temporary password (you would be asked to change it when first logging in).
Please email supportdos@larian.com, with a description of the problem and your system specs. I'm not sure if Yosemite moved anything around, but in Mavericks you could use 'system report' from the 'about this mac' apple menu item or the 'system information' app (utilities folder).
Also, please perform the following steps and include that information:
1. Verify your Steam game cache (if applicable)
2. Launch Activity Monitor
3. Start Game and get into a freeze state
4. Now Alt-Tab back to Desktop and check Activity Monitor. What do you see? How much CPU does the game consume?
5. Double click on the Divinity - Original Sin process and press 'Sample' button. Wait until Sampler is finished
6. Save sample output using 'Save' button
