If you have a noisy audio file, and you want to end up with a clean audio file, then
Audacity is probably better. It has a noise remover that takes an example of pure noise (what you want to be silent), and has some settings for how aggressive it should be. Too aggressive sounds "processed", not enough leaves some audible noise, and sometimes you have to compromise with some of each.
If you really want to use OBS, then you can set up the noisy file to play just like any other source, and click the Start Recording button. It'll take as long as the file is, but you'll get a copy of it in the recorded file. You'll have to stop the recording yourself as well. In between, in addition to getting the recording settings right, you'll want to add the de-noiser as a filter on that source.
