Case 4: OBS + OBS-ASIO (Windows only)

You are in case 4 if:

You uses Windows with ASIO drivers, whether using or not an audio software and/or an external sound card.

Check the version of OBS you are using: in OBS, click "Help" in the menu bar, then "About. The version number is just below the "OBS Studio" label.
Close OBS, then download OBS-ASIO. Here are the direct download link depending on your OBS Studio version: 25.x.x and above (preferred), 24.x.x or 23.x.x.
Then install the plugin.
Re-open OBS.
Click the "+" in the Sources window, and select "ASIO" to create a new ASIO source. Give it the name you want and click OK.
In the appearing window, choose your ASIO device:
- If you use an external sound card with a built-in loopback feature or if you physically hard-wired an audio input of your external sound car back into one of its inputs, then choose this external sound cards driver
- If you use two external sound cards, choose the secondary sound cards driver (the one in which you looped back your primary external sound cards output)
- If you created a virtual ASIO input device using VoiceMeeter Banana, Asio Link Pro or Jack Audio, ReaRoute, choose the corresponding ASIO driver.
Choose "Stereo" in the Format field
In OBS Channels 1 and 2 (stands for left and right channels), select the appropriate ASIO inputs you looped your audio back into.
Click OK then in the OBS Audio Mixer you should see the VU-meter of the created ASIO source moving relatively to the sound your audio software is outputting. If so, you are good to go to part 3!