How do I do an "or" orperation for a search in Apple Mail?
You can use Boolean operators:

Use Boolean searches:
Use Boolean operators such as AND, OR, and NOT. For example, to search for messages that contain “yosemite” and “yellowstone” but not “teton,” type “yosemite AND yellowstone NOT teton” or “yosemite AND yellowstone -teton.” You must type Boolean operators in uppercase.

Souce: http://support.apple.com/kb/PH14902