There are two folders that Windows looks at, to run things on startup: one for the logged-in user, and one for all users. Copy a shortcut into there, and it'll run after log in. I've forgotten where they are now, since I haven't run Windows for a long time, but they're in the structure that drives the Start menu. (The app launchers in the Start menu are actually shortcuts in a specific folder. A sub-folder of that is run on startup.)
Once you've made the copy, you can modify it to tell OBS what you want to do. obs --help in the Command Prompt to see all that you can do there.
