I have installed openHAB on my Diskstation using another way than the provided package. Mainly because when using the package openHAB had very poor performance on my system plus I wanted to have some more flexibility.
Short version: I installed Debian chroot on the Diskstation and then installed openHAB using normal Debian methods. I published a step-by-step tutorial here (English and German version available): http://internet-of-things.blog/en/installing-openhab-in-only-2-steps-on-a-diskstation/