Yes, you have to maintain 16:9 aspect ratio. If you just reduce horizontal, then your cropping vision without zooming the game out. In CSGO, black bars on the sides does nothing to the focal length. The whole point is to make it zoom out so CSGO and OW have the same focal length.
Example from a previous post. Both games had the same focal length, but different fov. The bigger the aperture, the bigger the fov. You can manipulate aperture downwards, but going upwards requires a larger monitor.