After a couple of days bashing my head against my desk, I have finally got a C client to talk to Drupal 7.x using Services 3.x (XMLRPC) and OAuth authentication. Previously I had been using Drupal 6.x with Services 2.x and an API key, but the API key option is not available in Services 3.x. I looked for documentation explaining how to set up Services 3.x with OAuth, but I couldn't find a step-by-step tutorial. When I finally worked it out, I decided I'd better write one so that others don't have to follow my path of pain.
Please leave any comments/questions below.