Yes, definitely remove oil residue before applying Ren Wax. Everyone generally loves Ren Wax on the forum, I see nothing but folks that rave about it.
I like it ok, it gives a nice appearance and some protection. But, in my experience, it’s not best for long term storage as I have experienced some light surface rust on guns stored with Ren Wax. I caught it time, so no damage, but it scared me enough to not trust it. I know how to apply was, and have used Ren Wax a lot in woodworking as well. My use of Ren Wax is when I want a gun to look good presentation/pictures and not have an oil residue, and I will also continually check those guns. BTW, I’ve experienced light surface rust with Ballistol as well, to me that stuff is useless compared to other, better options for gun oils/cleaners.
For long term rust prevention, Rigs grease is great, never a problem with rust. I’ve also had great results with long-term storage, believe it or not, with Frog Lube (I only use it as a protectant on surface metal, never as a lube or cleaner…). I use use “Corrosion X” as my go-to, everyday gun cleaning and wipe down oil. Works great, and I’ve never had any rust. Because of the hassle of removing oil and applying the Ren Wax properly, I really don’t use it much at all anymore, there’s better solutions, IMO.