Before we start... to stop your dialog needing scroll bars... with Sketchup closed Open the Strt > Run 'regedit' and find this keyHKEY_CURRENT_USER\Software\Google\SketchUp8\WebDialog_Octane
and delete it.
When you next run the Exporter within Sketchup it should be resized correctly...
Now for the problem...
Your settings seem OK.
If only exports the Textures of Materials in use, so unused materials textures are not exported.
Plain 'color' Materials are defined in the MTL file with no matching image.
If you click 'Export OBJ/MTL Only' you will get what you describe - plus an MTL file.
To export a render use 'Render Frame'. It should then make the equivalent files as before and start Octane.
Is the binary-path correct - notice it's to v1.2.3 - this exporter is for v1.2.2 - but it 'might' work [untried] ??