Google devices cannot be unlocked locally. My USA spec Pixel 5, purchased unlocked from Best Buy, had to be connected to the internet - and I don't remember, but I may have also had to put a SIM card in - before it could be unlocked. Apparently a background service automatically checks with Google's servers whether the device is eligible for unlock (is it paid off or still under financing, and which carrier is it from?) and then automatically fetches the unlock code if Google decides to allow it. But the default state until that happens is locked with no possibility of unlocking.

So it's very unlikely that you'll be refused the unlock on a device you just purchased, and if you are then you can just return the device, but I don't like the internet requirement since that means Google's unlock code service and all the other Google applications on the phone sent out a bunch of information about my device to Google.

When I tried initially unlocking the option was grayed out and unusable. I had to find information about the internet connection requirement online.

I don't know whether OnePlus does something like that - my OnePlus 7 Pro was connected to the internet before I attempted to unlock it, but I'm receiving a OP 9 Pro today and would like to unlock it as soon as I get it without connecting to the internet or putting in a SIM card. Will see how that goes.