Even with the entire boiler securely placed in a vice, I couldn't get the OPV to turn a nanometer (in either direction); I can only get the thin top to come off. Any ideas?

Beware... please read this thread on this site... Problems overhauling my Olympia Cremina

Edit 09:30...
I did some additional DD on this site because I thought there were a few other people that had the same problem. No luck...

I would be reluctant to give you any specifics because when dealing with a machine that may have been worked on by a novice at some point, they may have over tightened the valve. What they may not have known is that with brass on brass you do not have to get really aggressive with the wrench.

So with brass on brass torked on tightly it has formed a very tight bond. You might try PB Blaster as it works great. I might try to heat the brass at the joint a bit and then apply the PB and then let it sit for a day or so. When I say heat I do not mean torch it until it glows but just enough to get it up to 200 degrees or so and the apply the PB in good quantity so that it has the chance to try and seek any point to get in the joint. Then I may do it a few more times just to know I gave the PB a fair chance to work it's magic!

You may 1st want to loosen the sight glass nut to take any tork/pressure off the sight glass area. Be very careful in turning out that nut because glass is involved and those sight glasses are not easy to come by!

Good luck