What is an OS2SD ROM?
Judging from the name OS2SD (Operating System TO SD) and from the line that reads OS2SD friendly TWRP recovery, handles partitioning of sd card for OS2SD in this XDA post, it seems that it's a particular flavor of Android that's made to be installed on an appropriately formatted external SD. Aside from that, I was not able to get more info.

Anyone is invited to improve this answer with more info.