It is very easy to flash to CM13 on the OPO3. It is the same process as say a Nexus phone. Very easy set of commands just like any stock phone.
Here is the guide I used below.
http://forum.xda-developers.com/showpost.php?p=67318561&postcount=2
What did you gain by flashing CM13 vs OxygenOS? I am surprised they are that different. Did you lose features like in the native camera app?