AN894 fitting

I would like to install an AN894-D6-4 flared tube reducer to the bulkhead nut side of an AN833-6D bulkhead fitting. Currently it pretty much just bottoms out without getting tight. In Google searching it says an "O" ring must be used to get a seal. So where is this "O" ring suppose to go -
Over the bulkhead fitting against the bulkhead nut - - inside the reducer ?
In looking inside the reducer the base is chamfered as it would be to fit against the male end of a normal AN flared fitting so if the "O" ring goes inside what is going to keep it in place ?
On the other hand, putting the "O" ring in the threaded area of the bulkhead does not sound like a good idea so - - - -
Thank you very much,
Randy E.

don't use an oring. They are not appropriate for flare fittings. If you read the reviews at ACS, it appears this reducer is too big to fit most fittings. You need to either trim down the female side to fit (where it doesn't bottom out via the base) or install a new bulkhead reducer fitting to do the reduction.

Larry