Oooer, burn!
This phrase can be used when one person insults another person (whether or not they are present).
Kay: Hey Maura, yo momma so fat, she went out in high heels and came back in flip flops.
Maura: That's not very nice!
Kseniya: Oooer, burn!
Maura: That's not very nice!
Kseniya: Oooer, burn!