Aircrew Training
As long as you meet the qualifications requirements for Direct Entry (DE) Aircrew, you are able to attend OASC to attempt selection as a Serving Airman.
I am pretty sure that the upper age limit for Pilot/WSO is 32 for serving airmen and the upper age limit for WSOp is 39 (worth checking these as its been just over 2 years since I came through).
Something I am not sure of (as I transferred from another service) is whether Serving Airmen have to attend briefings and pre-OASC interviews at an AFCO.
Your Sqn admin staff should be able to offer you guidance as to which AP's hold the information you need.