What is the Noahide sin of cursing God and is it the same as taking God's Name in vain? Like, would saying "oh my god" count?
You misunderstand the usage of the term cursing God. It does not mean using bad language or taking the name in vain. The actual meaning is stating a curse upon the Deity using the actual name of the Deity.
For example, Is G-ddamn considered cursing God? explains:
The Talmud (Sanhedrin 56a) explains that the only form of true "blasphemy", as described in Leviticus 24:15, would then be to curse G-d in a biblical sense, i.e. to say "may Joe strike Joe" (or "may Joe damn Joe", for that matter), substituting "G-d" for Joe. (Which is a bit strange, if someone doesn't believe in G-d, why they're calling on Him to smite anything, but never mind.) Maimonides, Laws of Foreign Worship 2:7 discusses the severity of punishment may depend on exactly which name of G-d is used.