Question: is saying omg a mortal sin?
Answer: No, it's not a mortal sin. Personally, when I hear someone say "oh my God" or "oh Jesus" or a similar phrase, I reply "what about Him?" Whenever you profane the name of the Lord is His Saints, pray a quick prayer of reparation: "Blessed be the name of _________" (whatever name you abused). ... It is not a mortal sin.