OS Graveyard is the fourth stage of IT Catacombs, a straightforward boss after boss battle. Requires proper meatshielding to win.
- Professor A. appears after 2 frames.
- Bun Bun Black appears after 26.7 seconds800f
- Teacher Bun Bun appears after 33.3 seconds1000f
- Because this stage only ever spawns 2 enemies at most, Teacher Bun Bun won't always appear at the time stamp, but he appears when either Professor A. or Bun Bun Black is defeated.
Uber-less line up
- Mohawk, Manic Mohawk, Eraser, Manic Eraser, Prisoner, Crazed Bahamut and Dark Emperor Catdam.
- Demonstrated Video: 
Max up power up. Use meatshields to stall until you have enough to spawn Megaphrodite. Stall black Bunbun with Bombercat and meatshields till Megaphrodite kills Prof A. Have Necrodancer and any other cats which freeze flying enemies. Use Windy or A. Bahamut to finish.
Strategy 2 - Uberless by MantiCorgi
- Mohawk, Any form of Crazed Cat, Eraser, Roe Cat, Bombercat, Necro-dancer Cat, Hacker/Cyberpunk Cat.
Upgrade your Cat Wallet to 2, then spawn a Roe Cat. It can hold off Prof. A until Bun Bun Black spawns. By then, you will have enough cash for a Hacker/Cyberpunk Cat. Have Bun Bun Black kill the Roe Cat, then spawn Cyberpunk Cat, your 3 cheap meatshields and Bombercat. As soon as Bun Bun Black is perma-frozen, stop spawning Bombercat. Hacker/Cyberpunk should be doing damage to the Prof. A. When money allows, spawn in some Roe Cats for defense against Prof. A. in the situation when Prof A kills your BomberCats, just repeat the process. Eventually, Prof. A will die, so Bun Bun White will come out. There, spam Necro-Dancers to permafreeze them. Congrats, you should be able to either kill both and progress, or completely defeat the tower.