While Roy departed for 82, the latter slammed a blistering ton.
Bairstow later added 151 runs for the second wicket along with Alex Hales.
Hales too broke the shackles, finishing as the highest run-scorer (147).
Eventually, skipper Eoin Morgan's 30-ball 67 propelled England to a ground-breaking total of 481.
The records that were broken during the ODI
England registered the highest score in ODI history, beating their previous record (444/3 vs PAK, 2016).
They also became the first ever side to reach the 450-run mark in ODIs.
The run-fest came 11 days after New Zealand made the highest total in a women's ODI (490/4 vs IRE).
Notably, Morgan became the first batsman to score 3,000 ODI runs in England and Wales.
Australia were bowled out for 239
The Australian top order settled initially, however, the pressure of chasing a mammoth total derailed the team.
Travis Head (51) and Marcus Stoinis (44) made useful contributions, but failed to capitalize.
For England, Adil Rashid was the pick of the bowlers, who finished with figures of 47/4.
Moeen Ali claimed three scalps, while David Willey picked up two.
England's biggest ODI victory
The hosts trounced Australia by a record 242 runs in the match. This was England's biggest ODI victory with Australia suffering their heaviest defeat in terms of runs. The two teams struck as many as 62 boundaries (most number of boundaries in an ODI).