I have been away from SIGs for a while now, but I am back to shooting and enjoying them. I would like to carry my P320C but I am having some difficulty finding an OWB holster. Is this gun the same size as some other SIG-Sauer? P250 maybe?

The P250 and P320 are identical for holster fit purposes. Their built around the same dimensions, and even share a number of parts. They just have some major internal differences.

In your case, your P320 Compact/Carry will fit in P250 Compact holsters.

There are a large number of holster makers out there with P250/P320 molds.