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Not sure about SLOBS but here is a quickstart guide to run Diablo on OBS.
Note this only works for the "DX MODE" version

-Launch DX Video Settings
-Change "Presentation" to Windowed
-Adjust "Display Mode" to your personal resolution preferences
-Exit the settings, Launch Diablo (DX Mode)
-For OBS it should be as simple as tabbing out of the game and capturing it via "Window Capture"
-The game does upscale only in 4:3 so if you may want to set up some cropping when game time comes!