I have been building my own custom 'Advanced Search' page and I have had some success in getting it to work. Basically I understand that when searching on tags and such that I can do an "AND" search by doing something like:
This will return all posts that are tagged with apple AND with orange...but leave out those tagged with one or the other only...or not at all.
I was reading a bit through Google searches on this subject and I came across this website: http://wpguy.com/articles/multiple-tag-category-queries/
which claims that one can do an OR search by separating the tags by commas, e.g.:
This should return results that are tagged with apple, oranage or both. This I cannot get to work though. Is there a way to do this through URL arguments or not?