Basically you should watch sotus (and sotus s) before watching Ourskyy. Sotus provides the background context for one of the episodes and is the backstory of their whole relationship. While you can watch them out of order, I would highly recommend watching sotus first: it's way cuter knowing all the background information. The episodes are all on youtube with english subtitles

Spin-off episode of PeteKao, basically like a side story (the episode) to Pete and Kao's antics....it's a super sweet ad-on to the GMMTV universe.