Updating the Adblock Plus extension fixed it for me. There is an "update now" button in Adblock Plus extension's options, under "filter lists" tab.

Figured it out!

Turns out the AdBlock Plus extension was causing the problem.

I got the same problem.

Looks like it's loading youtube without css.

Opera Version: 34.0.2036.47
System: Mac OS X 10.9.5 64-bit