How do I beat O/S?

Way of White and Warriors of Sunlight covenants have a resistance to invasions, but it's mostly a matter of luck. The real issue with getting to the fog door tends to be the giant Royal Sentinels on the ground floor next to it.

Alternately, camp out in the bonfire room near all the Silver Knights (where Solaire was) and try to summon a player ally _before_ running off to the boss. It may take you 10-20 minutes to do it successfully but they'll typically be much better at fighting the boss and also save you the hassle of running all the way around the rafters to reach Solaire's sign.