It has been long discussed topic and there have been lot of attempts in this direction.
I came across this website and had a glimpse of what kind of an online OS can be. It contains a 3GP playerr, mp3 player, calendar, WYSIWYG HTML editor, calc, messenger, forum and an inbuilt browser – more than anyone can expect so many apps at one place. Voila! here comes a great attempt in creating an online operating system.
Although it is very slow and many things look pretty clumsy but definitly you will like to have a look at it.
Way to go eyeOS!