There was a popular band back in the 90s named Oasis. They look like the two in the bottom. (You'll probably recognize them most for the song Wonderwall)
This joke is both a straightforward pun of their name being Oasis but also a little bit of a deeper cut because the two members of the band fucking hated each other the entire time so them being together has to be a mirage.
A mirage is an optical illusion, very commonly seen in the desert, and often portrayed as a wonderful water oasis when a person is dehydrated and lost. Of course, the band Oasis isn't standing in the desert either, so it must be a mirage too.