“I heard in a shiur from someone who knows his stuff, that sayin ‘Oh my G-D’ is ossur mideoyrayso.”
According to many poskim, yes.
Sister Bear:
“I learned in a shiur and saw inside the Torah and meforshim that adding on to the Torah is an issur deoraisa.”
True. But I’m fairly certain that “lo sisa es shaim Hashem Elokecha la’shuv” was part of the Torah ever since Hashem gave it to us on Har Sinai.