The letter sequence <ost> has a number of different pronunciations.
1. ɒst. Examplesː
Boston cost costermonger frost hostage hostel lost nostalgia roster
2. əʊst. Examples:
ghost host most post postage
The suffix -most is also has this pronunciation. Examples are:
foremost innermost topmost
The word posterior has ɒst for some speakers and əʊst for others.
3. ʌst only in the archaic word dost, which is the second person singular simple present of the verb do.