I just added an OctoPre Mkii to my recently new 18i20. Perhaps I am doing something wrong, but I can't get them to lock at 48khz.
The Mix control panel shows 48 and internal is locked, but the OctoPre's lock LED only lights when set to 44.1 (wordclock is lit in either case/rate).
 
This may be a case of having older Sonar projects which I did at 44.1. Now I'd like to start doing things at 48khz.
 
Michael

How do you have the two units physically linked? 
 
the wordclock input and output of the 2 units need to be connected for syncing.
 
http://www.manualslib.com/manual/626077/Focusrite-Scarlett-18i20.html?page=18

Yep both optical and BNC cables.
Here's what I've found so far.
With a new project, both will lock at 48khz.
With old (ie 44.1)ones, the MixControl pops back to 44.1 and though it's window says locked and sync'd, the OctoPre falls out of lock.
So, it's me! LOL    
Thanks Beagle. I now have sync/lock at the project's correct rate, if I pay attention. Thanks for the link.
Michael