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Arncott is a chapelry of Ambrosden, so their parish records are included with those of that parish.
Ambrosden baptisms (1611-1818) can be found on the Family Search website (Batch No. C024102). Although there are several BUSBY baptisms there between 1775 and 1809 (various parents), there's no sign of your William.
Neighbouring parishes to Arncott are Piddington, Charlton-on-Otmoor, Ambrosden, Merton and Blackthorn. It should also be noted that Arncott is right on the county boundary with Buckinghamshire. It may therefore be useful to continue your researches in these parishes/county.
I did find the following baptism at Charlton-on-Otmoor (on Family Search):
23 JUN 1810 - William BUSBY, s. John and Ann
and this one might be worth investigating further.
Pennie