In those cases on which you get empty directories is because the files for those packages were never branched into the oacs-5-6 branch. So CVS replicates the directory structure but doesnt download any files. So far you will find only core packages under the oacs-5-6 branch.
So you are not doing anything wrong is just that there are not files for those packages under that branch. So you can use the HEAD version for those missing in the oacs-5-6 branch.
Oh great, that is much clearer now. Thank you for the quick reply Victor.
I guess branching packages is the responsibility of each of the package maintainers, and if they haven't actually tested the code with the new core, it is not logical to branch.