If I remember correctly, McVay did not allow Kevin O'Connell to join Staley with the Chargers.
McVay was able to block him because it technically would have been a lateral-move. However, it would have been an up-grade in the sense that McVay is essentially the Offensive Coordinator with the Rams, and O'Connell is his assistant.
Similar to when Matt LaFleur left to be Vrabel's OC in Tennessee. That was also technically a lateral move but McVay (graciously) allowed it to help LaFleur's career; and a year later he was the Packers' Head Coach.
I was impressed when McVay allowed LaFleur to go to TN. Thought it spoke well of him. Understood him wanting to hold onto O'Connell given all the good young talent he had lost off his staff in recent years. Still, should O'Connell get an opportunity to be the Offensive Coordinator for a head coach with defensive-background, I could see McVay letting him go in 2022 ... and maybe Thomas Brown would get promoted to OC.