Writing letters did the trick for me when I was hoping to visit a few places prior to attending OASC - I wrote to the station commander, and hoped it would be sent to the appropriate people. I wrote to Leuchars, Cottesmore and Waddington, and was invited to the latter two bases, though obviously it took a while for the letters to be passed through to the appropriate people (in my case people in Int/Ops), it worked out very well and I was pleased with the tours, advice and information I was given!