18mm osb is fine but it needs installing correctly
sole plate needs securely fastening down then racking board needs to start with a full board stood up right, nail at the correct spacing and catching the first 3 studs, then repeat, don’t end up with a skinny bit on the corners, you should have full boards on the corners working away from them.
If you have any internal walls dissecting a long run of external wall it might pay to turn that into a racking wall, you can just board the first 1200mm on the internal one and add strips to bring your plasterboard out flush.
I take it you have no drawings for this just whinging it a bit, I’m no structural engineer but I have put up one or two timberframe houses so just going on what I had to do in the past.
Have you goy the trusses up yet.