Conditional Comments are an IE-specific feature, and will be ignored by opera etc.
They do support "or", however in the form of a pipe like so;
<!-- [if (IE 6)|(IE 7)]>
this is only seen by IE 6 and 7
<![endif]-->
source: msdn
EDIT
As commented, it is indeed preferable to write cross-browser-compatible CSS when possible, and use conditional comments only as a last resort. To make life easier, be sure to avoid quirks mode and use feature detection over user agent sniffing. Check out the modernizr library which helps with the latter.