Not usually from Heathrow to Oxford. Occasionally, on peak time departures from Oxford to Heathrow, they tweet to advise that unreserved passengers join at the Oxford Gloucester Green bus station where the service originates, rather then try and board en-route.

Unreserved passengers joining at the LHR Central T2/T3 bus station have an advantage - as the service starts there, they get first shot over unreserved passengers joining later on at T5.

"The bus will usually turn up at the expected time". The Oxford Airline bus is one of the more reliable public transport operations, but there are very occasional mechanical cancellations and delays when the inbound is delayed on the motorways to and around Heathrow.