Put a shortcut to OBS in the startup folder:
Win 8/8.1:
open the start menu/tile thing and type in:
Run
and hit enter. Then type in
shell:startup
and it will open the startup folder. Put the OBS shortcut in there.
Alternatively, go to the start menu and type in
%appdata%/Microsoft/Windows/Start Menu/Programs/Start-up
and put the OBS shortcut in there.
Win 7:
Open the start menu
>All Programs
>Startup folder should be there.
Put the OBS shortcut in that folder.
This is true for all programs you want to start on windows starting, not just OBS.