In message , Tim Watts
writes

The frame is not crooked - so it should sheet up nicely. And it's not
falling down - it's just not as good as it should be.

OSB visually is not a problem - this would be further covered by extra
celotex then plasterboard.

If anyone can explain "structural ply" I'm still interested though - for
other reasons

Bit here....

http://www.plywooduk.com/ceMarking.pdf
--
Tim Lamb