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.
Bonus panel: There's a third guy, he sees The Line (the planned city that's basically just a wall through the Arabian desert) just over the horizon, he stops, looks back at Oasis and shouts "hey, play Wonderwall!" and starts crawling towards salvation. (Which never comes, because it's all a mirage).