Google Search Operator – site:example.com this OR that

If you specifically want a logical [OR], Google does support use of “or” in queries. In this case, you’d get back any pages indexed on the domain that contained either “this” or “that” (or both, as with any logical [OR]). This can be very useful if you’ve forgotten exactly which term you used or are searching for a family of keywords.
Edit: Hat Tip to TracyMu in the comments – this is one case where capitalization matters. Either use “OR” in all-caps or the pipe “|” symbol. If you use lower-case “or”, Google could interpret it as part of a phrase.