"At the first possibility" and "at the first opportunity", are those interchangeable? Is option 1 grammatically correct?
Option 1 is not correct. The two words do not mean the same thing and are not interchangeable.
An 'opportunity' is a specific time, occasion or situation in which something can be done.
A 'possibility' is a chance that something could happen.
For example, if someone asked you to do something by the end of the day and you knew that you had the time and means to do it, you might say "I have an opportunity to do it". However, if you were not sure whether or not you had the time or means to do it, you might say "there is a possibility I could do it". You could even say "there is a possibility I might have an opportunity"!
Having said this, the word "chance" can sometimes mean either opportunity or possibility, and this could be interchangeable. For example:
- I haven't had the opportunity to do it
- I haven't had the chance to do it.