As far as I know, UCSF is the only GME program as of now that requires Step II CK and CS be passed for rank list, for any specialty. I'm not aware of any other rad onc programs requiring this at present.
The UPenn IM program flirted with a similar requirement this year (don't recall if this was CK only or both CK and CS); unclear what its fate will be for the future and impact on rad/onc. IIRC the integrated medicine prelim year at Penn Rad Onc was exempt (but someone in the know correct me here).
October is plenty early if you pass on 1st attempt (read FA for CS even if you're God's gift to the clinics; most AMGs fail due to not knowing what the test is looking for) - result should be back around Dec-Jan. If UCSF is not on your list, this is not an issue at all.
Good luck!
P.S. This question did not come up on any prelim year interviews for me. I'm sure they would typically want you to have passed it before you start though.