Further info:
If I disable the use of Ajax in the Configuration for OE then all appears to work. This, to me anyway, points to a problem with the module referenced in the error message below.
Parse error: syntax error, unexpected '?' in /home/mysite/public_html/osc/admin/edit_orders_ajax.php on line 1028
BTW, I'm using Firefox 3.6.3 and I've tried this on a couple of different computers.
John