If you want to record a local file without black bars, set the canvas and output resolution in Settings > Video to the same resolution as the native resolution of your display. Even if it is not available, you may enter the correct values directly into the field.
HOWEVER
Services like Twitch, YouTube and others assume everyone has and wants a 16:9 aspect ratio. As a result, any presentation of your videos to those viewers, if presented full screen, will present with black bars.
Since your aspect ratio is different from the audience's presumed ratio, a correction has to be made. This can be done with black bars at the top and bottom (letterboxing) or on the sides (pillarboxing) or else the video can have its top/bottom or sides cropped, or the image can be stretched or squeezed in order to fit.
If you record your display with its native settings, your audience will see one of these transformations. The only way to prevent them from seeing this, is for you to see it on your end by forcing OBS and your applications or display to use a 16:9 aspect ratio, and adjusting your sources in OBS to eliminate or compensate for letterboxing/pillarboxing or stretching. Which of these is needed depends on what resolutions your monitor supports, what resolutions the software you are recording supports, and how it behaves when you try to force it to display in a non-native resolution on your display.
