|Game: Sonic the Hedgehog 2 (16-bit)|
|Level: Oil Ocean Zone|
|Hits to defeat: 8 (for the main body), 4 (for each cannon tentacle - optional) - 16 total|
Mega Octus is the ninth major boss fought in Sonic Mania, encountered at the end of Oil Ocean Zone. It resembles a large version of the Octus badniks found throughout the Zone, and shares many similarities with Submarine Eggman II, the boss of the original version of this Zone from Sonic the Hedgehog 2.
The boss arena begins as a row of three large platforms with bridges between them, suspended over a large pool of oil. The battle begins when two robotic tentacles emerge from the oil and wrap around the leftmost and central platforms, pulling them beneath the surface and destroying the bridges between them. If the player is standing on the platforms when they're pulled under, they will be pulled beneath the oil to the bottom screen boundary and lose a life, though this can be avoided by jumping before the platforms reach the bottom of the screen.
Once the platforms have been pulled off-screen, the main body of the Mega Octus will quickly surface from beneath the oil, and Dr. Eggman will pop out from the hatch on top of the machine. The Mega Octus will remain above the oil for a few seconds, after which Eggman will close the hatch and the boss will quickly dive back down beneath the surface. While the boss is above the surface, the player can damage it with a Spin Attack. Once the main body has submerged, the platforms it pulled beneath the oil's surface will rise back up into their original positions.
Once the platforms are all back in place, similar to Submarine Eggman II a detached tentacle with a large spiked tip will launch up out of the oil in an arc, jumping over the platform the player is stood on before submerging beneath the oil's surface. Like the original, this will damage the player if touched, but can be avoided simply by jumping over it.
Next, as with Submarine Eggman II, a tentacle with a laser cannon on its tip will rise vertically from the oil. Just like the original, the cannon will turn to target the side of the screen the player is on and fire three shots towards them at varying heights, which can be jumped over or ducked to avoid. As before, if a shot skims the top of the arena's platforms they will ignite on contact, creating a wave of pink flame the player must jump to avoid. Once three shots have been fired, the arm submerges beneath the oil again.
After the cannon submerges, four tentacles will emerge from the oil. Two of these have spherical cannons on their tips and mimic the antennae-like cannons from the Heat Arms boss fought in Sonic & Knuckles' Lava Reef Zone, rising vertically from the oil and shooting projectiles diagonally towards the player. Each of these tentacles can be defeated by hitting the spherical cannon four times, though destroying them is not necessary to defeat the boss. The other two tentacles will wrap around the two platforms nearest the player similar to the beginning of the boss fight, pulling them beneath the oil after a few seconds.
Once the platforms have disappeared, the main body of the Mega Octus will re-emerge from the oil and the cycle will begin anew. If any cannon tentacles remain, they will submerge whenever the main body dives beneath the oil and reappear each time the body is about to emerge unless they have already been defeated.
Landing eight hits on the main body will cause it to explode and sink beneath the oil, along with any cannon tentacles the player has not yet destroyed. Once the boss has been defeated, any submerged platforms will rise back up from the oil, after which the player can proceed to the Capsule and complete the Zone.