You might not know that even though he mainly participates in Onikakushi, you can actually fight Ooishi as a secret boss in Watanagashi. When you learn about the most recent disappearance from Oishi, instead of going back into your house, enter the car again. Oishi will be surprised at first, but then decide that you're a discerning individual

and request that you take his secret. He'll then reach into his car AC and remove the LoadedFilter. Hold onto this until the scene where you enter the Sonozaki Basement. Instead of directly following Mion, there will be a moment where you can continue down a side branch and find the Sonozaki Basement security system.

Insert the Loaded Filter into the AC system and Ooishi will materialize, taking over the entire location. After cackling about achieving his true power to escape the game board, he'll realize that he's still too limited to reach the power of a true Witch...

only to realize that Keiichi's "Magician of Words" title is the last element he needs to become a true magical being and reach the Sea of Fragments.

Just when it seems like he's going to destroy you instantly, Mion, Shion, Rena, and Okonogi will burst through the door and assist you in the ultimate battle. After beating him, (if you can, anyway...) you'll get another piece of Shadow Candy which lets you get the Lambdadelta route in the Answer Arcs. Just be careful...