The point of putting the OSB3 vapour-breatheable but airtight sheathing/racking, all edges supported, gapfilling glued and screwed/power nailed, on the outside of the studs, rather inside as is universal timber framing practice, is that in that location it's uninterrupted by joist ends, partitions, service holes etc, all of which have to be carefully sealed around with expensive sticky tapes, which professional airtightness testers know are rarely done perfectly and may start parting within as little as six months. Avoid all that by putting the OSB3 on the outside face of the studwork i.e. in the middle of the insulation sandwich - it's then stupidly easy, foolproof to do by ordinary chippies (see pic). With plasterboard direct on the inside face of the studs (which is OK to puncture for services, so no service void required) and OSB3 on the outside, no problem with restraining the insulation between the studs.