It depends if you mean online like here or online chat in gmail, facebook etc. When chatting I tend to behave pretty much the same as in real life. I tolarate longer pauses though.
If you mean this kind of online I give my opinions more freely via a computer.
I'm FEEL more open with deep thought, and I can articulate my thoughts better only because they're presented in writing.
The key word is feel; I'm not sure that I AM more open online as I often get the lazies ("I COULD think this out and put it into writing via two large paragraphs. . . Or I could not ^_^"). Obviously the latter isn't the issue in person, and the biggest hinderances are lack of eloquence and political correctness.