You need to upgrade any occult weapon to +5. You can use the Occult Ember or the Occult Club (dropped by the mimic in the room with Havel's set in Anor Londo. The Occult Ember is found in the Painted World of Ariamis (use the Annex Key and follow the straight path after opening the door. The ember is down the stairs and on a statue that looks like Andre of Astora). If you are using the ember, give it to Andre and use it to ascend any +5 divine weapon to Occult. I'd personally recommend using the club so you don't have to waste resources.
To get an Occult weapon to +5, you need 9 white titanite chunks (I'd recommend farming the axe wielding black knight at the end for them) and a white titanite slab. White titanite slabs are an incredibly rare drop from the pinwheel servants or you can find one in that area. Describing where it is will only confuse you, so see this videofor the location.