I got the Troy Delta rail and it really is drop in. I actually was having an issue at first and called their technical assistance and they sent me a new piece because they had a few that weren't in spec. Once I got the new piece it took all of five minutes to install. It should fit your OR fine, it is free float and has a hole for your front sight or in your case, gas block. I like it because it enables me to reach my support arm out farther.
https://troyind.com/products/delta-carbine-blk