They certainly can be. Is it common? No. But there are examples out there. Joe Brady was a night and day difference after he took over OC for the Bills. Josh Allen's career also turned upwards.
The Chargers with one of the worst defenses last year, are one of the best this year under Jesse Minters, largely with a vast majority of the same players.
Even Bryce Young looks like a different QB under this new OC.