Op5's Normal os is ok .I dont think the custom would run as great as oxygen os.Moreover the oxygen os is a custom version of lineage os.
Please stop distributing false information, as I said in a previous thread OxygenOS has nothing to do with Lineage.
OP, can't help you personally as I've left my 5 stock (think it's the first android phone i haven't unlocked...) But I can tell you that you're best going to XDA and the official Oneplus forum where they have specific unlock/recovery/ROM/root forums.
I can tell you from experience that Oneplus phones are some of the easiest and best to mod, and if there aren't already there will soon be too many good Kennels and ROMs to choose from. They release their kernel source and device trees pretty freely, one of the reasons i like em, I'll always be a tinkerer at heart