This may be the the compression fitting @TinWhisperer is referencing.
Not sure about the o-ring end. You might replace the oring all together. Maybe put a nipple and adapt from there.
I went to a race shop and had them make me a flexable hose (break line) to connetct to a fitting from the compressor to the derale AN8 fitting. I ran copper tube from the water separator to the tank and used the same compression fitting from mcmaster carr