Is it permissible to say "oh my god" and is it considered an act of disbelief in the Beautiful Names of Allah, given that the word "God" was not mentioned in the Qur'an or Sunnah?

There is no harm in using the name "God" for Allah Almighty, especially if this word is part of the speaker's language or if there is a need to use it. However, it is better for a Muslim to use the name "Allah" with its Arabic pronunciation in their acts of worship, supplications, and in all other circumstances, because this name has become a distinctive mark for Muslims, and it helps to avoid confusion between what is meant by "Allah" and what is meant by "God," which may sometimes be used for other than Allah.