I have just bought an Apeks regulator and had it put together. I then bought a BCD separately and want to attach the inflator hose to the regulator. My inflator hose does not have an O ring at the end where it attaches to the regulator.

Yes, there should be an O-ring. I would go back to where you bought it, and request a new inflator hose with an O-ring.

If you bought it online, good luck. If you don't have any luck if you purchased it online, head to a dive shop and they should have an O-ring to fit it. The thread size is typical on the inflator hoses, so they should have extras.

Scuba Diving for Beginners : How to Use Scuba Diving Regulator