I've built some 2-metre square barn doors which are like this:
http://wiki.diyfaq.org.uk/index.php?...FaceOfDoor.png
but faced with 18mm OSB3 board.

Sagging is not a problem, but some of them are a bit twisted. In other
words, when they are closed, the bottom edge is flush with the frame at
both ends, but one of the top corners sticks out a bit.

Any advice on how to fix this? They are very heavy, so I don't really
want to take them down and rebuild. I was thinking of forcing them back
to the correct shape, and then fixing a steel or wood batten across one
of the diagonals to hold it flat.