Really appreciate your advice but I don’t understand what you mean - tap the plug through? And re the countersink do you mean ensure the head of the screw is buried in the wood?
And about the 5mm plug, I was gonna fix the new osb slightly lower as I’d inadvertently drilled into very old and pretty much perished mortar. So hope not to have to re-use the hole, just filling it for aesthetics.
Sorry for not being clear.
I meant that if you are going to use the fixings that you linked to, you can tap those in from the front of the OSB without needing to remove the OSB. If you do that though, you will need to countersink the holes to hide the fixing's nylon collar (and screw).