A UUIDv1 contains the host's MAC address as a component. And the uuid-ossp extension (or rather, the OSSP's libuuid, which it uses) does indeed read said MAC address when building UUIDv1s: https://github.com/sean-/ossp-uuid/blob/master/uuid_mac.c
Keep in mind, if you're using uuid-ossp rather than just relying on Postgres's built-in gen_random_uuid() function, it's very likely because you want to generate UUIDv1s instead of UUIDv4s (as they're the only two types of UUIDs that see much use.) So this functionality is pretty essential to the extension—a version of the extension without it wouldn't be worth much.
Keep in mind, if you're using uuid-ossp rather than just relying on Postgres's built-in gen_random_uuid() function, it's very likely because you want to generate UUIDv1s instead of UUIDv4s (as they're the only two types of UUIDs that see much use.) So this functionality is pretty essential to the extension—a version of the extension without it wouldn't be worth much.