I have a couple of hand-made leather holsters custom made to the guns I carry in them(I actually give my holster maker the gun, and he molds the holster to it).
Without actually being a "retention" holster these do a pretty darn good job of retaining the gun without any additional straps or other fittings-they pretty much require you to pull the gun straight out. I do have a thumb break on some for better retention. The straight sides of an automatic make retention even better than on a revolver.
A well-made leather holster is truly a work of art. I have some plain ones and some with various tooling patterns. One of my favorites is what's called a triple basket weave with my initials and made for my Colt Trooper.
I open carried for a long time in KY before I got my CCW, and still do with some regularity. For non-LEO, I think a true retention holster is probably overkill, but I do think some level of retention to prevent a random gun grab or more importantly the gun falling out is a good idea. If nothing else, you should look for a strap, although I have a strong preference for thumb breaks for their easier operation.
If you've been carrying concealed for a while, you likely went through a bunch of holsters to find one that suited you, and probably still select from a couple depending on the situation. Open carrying is no different. I've found that a right hip holster with a small amount of forward cant works best for me, but again that's a choice thing and I do have them in other configurations as the situation dictates. A shoulder holster can be a very comfortable alternative with bigger guns and in certain situations.