I'm not exactly sure what happens here, but if I set the project to linear instead of gamma (default setting in build settings) the lighting looks wrong in the Octane Viewport.
The thing is that the gamma workflow is not viable anymore when developing anything else than mobile applications, I'm not sure why unity still set gamma by default (maybe to preserve compatibility with mobile devices) as it renders unrealistic lighting in unity games.
My guess is that octane is doing a double gamma correction on the texture lookup when lighting is being set to linear.
I was experimenting with octane on my arch viz projects, but my projects are working in linear, I'm now sure if it's going to be compatible.
It would be nice if Octane could keep a consistent look when using both lighting settings.
Additionally, there are no information regarding Gamma vs Linear in the Unity Octane documentation.