No, osteopaths are Primary Care Practitioners which means they are qualified to diagnose and treat problems with the musculoskeletal system. It also means that they are qualified enough to look out for Red Flags and to know when to refer to an alternative practitioner such as your GP. Because of this you can book straight in with any osteopath without needing a referral.
If you are undergoing treatment for anything under the supervision of your GP, it is always advisable that you let them know you are intending to visit an osteopath, just so they are aware of the situation.