If he is experienced enough (w/ that big of a pond I'm sure he is) then he can do it w/o harming the fish, and as long as the handling is fast and properly done its better to do that to large fish then more fragile small fish
If you gotta do it there is a couple different ways to do it safely, you could tranq it w/ clove oil (you would need to figure out the best amount, I probably wouldn't use this method unless it was a study and I was measuring a ton of fish), you could put it in a very shallow amount of water w/ a proper ruler (water safe) in the water so it can breath, or my favorite is just wait until its up against the glass
About the Oscar I would just wait to move him until you know just by looking at him then comparing him to your other fish that he will be safe. I've found that's the best way to transfer fish, just wait until its obvious lol. I used to do the same thing and got some fish killed thinking they would be fine
^ that was a 4 inch heckelli threadfin. I measured him then thought, well a 5-6 inch wolf fish won't kill a 4 inch arcara.. he was fine for a week then I woke up to that. I know you love your fish man, I've lost a lot less fish since I just started waiting until it was obvious they'd be okay
Another method of measuring, you can put it in a separate tank, use some sort of sheet (plexiglass whatevs) to lightly force the fish right up against the glass w/ a ruler already there and the camera waiting. That's how I've seen scientists document specimens while doing research in the wild
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