Plus one for Envirograf - they will help you if they can they will ask you for such things as the surface material - in your case OSB, the fire rating needed and the area to be covered, they then work out the amount of paint and sell you that amount - they won't sell you less and will specify the number of coats of paint. Then once you have done the job they will send you a certificate you can shown to building control. It feels like a faff but its certified work you can actually do yourself. We have used it on all the structural metal work in our build - very limited amount to be fair. They will even give you stickers you can put on the finished surface warning people not to strip it!