It's quite common for the easy overtime to be taken away, especially in management changes. Sounds like he knows you're fleecing them and doesn't like it. Even if you opt out he'll just refuse to authorise your OT. Can't say I disagree to be honest, it's rife at our place. This "has to be done OOH, it will take a full day". When in most cases it can be done during the day and will take two hours max.
As much as I wish this was the case, it really isn't. Our OOH work involves putting an entire building in darkness, working over desks, shutting down critical plant, etc. I don't think a company which pays £1m a year per floor would appreciate us working on a tower above them on a weekday.
The fact is that the client pay for this work, it's part of their contract. They acknowledge that this work can only be done OOH and they pay OT rates for it. It's got to get done, one way or another. The only question is who does it: us or agency staff.
This isn't cost cutting at all.