Plurals

What is the correct plural for these?

Hyppopotamus
Octopus
Platypus
Omnibus
antenna

Hyppopotamuses or mi
Octopuses or octopi
platypuses or platypi
omnibuses or omnibi
antennas or antennae

For bugs, the plural is antennae. For radio antennas it's, as I just wrote, antennas.

all I know is that octopi is correct, but almost no one says this, most people just try to steer clear of using the plural of "octopus"

Octopuses is fine. Octopi was an attempt to Latinize the word. Octopodes is a popular choice for many. There's a good thread about this in the archive somewhere.