The most common cause of this kind of report is modeling some face 'backwards'.
To see this use a SUp Style where the back_material is say 'bright blue' and then view your model in Monochrome Mode.
All materials will vanish - everything should be showing in the off-white front-material [default] - those faces showing blue are 'reversed'.
The Exporter does what it is told and the faces' front materials are
exported and applied to the front faces in Octane. Octane makes all rear-faces the default-material. Unfortunately if you have added a material to a back that's facing the wrong way it won't be rendered: the rendered face is looking the wrong way.
It's a problem with 'sloppy modeling' rather than the Exporter or Octane.
To resolve it you need to flip [reverse] every wrongly oriented face and it's material from the back onto the front - this is a pain - especially if there has been texture mapping done. It really is best to get the model right before texturing and Exporting...
However, all is not lost... I wrote some tools to help - they are included with in the more recent zip versions and there are instructions at the end of the Help PDF...