This answer to this question led me to a good answer that I don't believe is a complete solution, but it works. if your tag are
C, you would type into the search bar:
Your particular solution will be
That is, to AND them, include
%20, and to OR them, include
%20OR%20. The problem with the current situation is that the search bar currently replaces
, a space, with
+ before searching. Therefore, when you type a search, all the words are ANDed together.
%20 represents a space in a URL. I don't believe this will change unless the search bar is modified fundamentally.
You can, however, get this functionality in a search bar by going to https://stackoverflow.com/search and using the Google search box. Typing
user-interface android OR user-interface ios yields this page, which contains all the questions you're looking for.
EDIT: It appears from this blog post that you can actually type spaces into the address bar, as in
https://stackoverflow.com/questions/tagged/user-interface android OR user-interface ios
rather than messing with the kludgy hex code. I would like to see this changed in the way that the search box works.