Green’s Dictionary of Slang

oh mamma! excl.

(US) an excl. of wonder.

[US]T.A. Dorgan in Zwilling TAD Lex. (1993) 60: (TAD’s Impressions By Reading Account Of Walcott’s Victory Over Choynski) Oh mommer.
[US]S. Lewis Babbitt (1974) 97: But, oh, mamma!, what conversation.