If by "English" you mean "US-English" (as most of the world do) then you should take into account fact it already is a very mangled and simplified language due to the sheer amount of people that talk it as "second tongue" even in the US.
A couple of centuries of continuous immigration from the farthest corners of the world produced a language that is much simplified (with respect to the "original") and easy to use mainly in everyday exchanges, but lacks in depth.
As an example: all Philosophical discussions are done using German words.
The major anchor to keep a language fixed are the recorded fonts.
Without such anchor languages drift quite easily (and fast!), especially if groups are small and with infrequent contacts.
Some expressions lose completely their original meaning; the cited "oh God!" might have become a generic interjection with no link to the Almighty (and probably spelled as "oggod!").
This happens continuously in "live" languages and especially so with US-English which is open to all influences.
One example also here: "placebo" original meaning ("I take the place of") has been forgotten to the point it acquired a "beneficial" role, so much so a new word was coined to express the "negative counterpart": "nocebo".
In general, depending on the amount of book/video which survived apocalypse (and ability yo read/play them!), you may bet on a further loss of many words not representing something some aspect of the current life and birth of specific "slang" words to detail something important for survival (Inuit don't have a word for "green", but have more than 20 to represent different kinds of ice).
Some words, now common, could become used just in fairy tales for kids.