Timely - I discovered OBS only last week, and am consistently amazed by it. Only thing missing is the ability to draw crop areas prior to a screengrab/video.
I'm not certain what you're needing but if you want to make changes to a scene before making it live you can enable studio mode, this gives you a preview of the new scene before you actually switch to it
If it's literally just cropping you're after hold alt while resizing the element
I feel like I've seen this. Can you check stuff like capture settings?