I found another problem. I've used TIG's exporter for years so tons of my obj geometry have been exported this way. It works perfectly well.
What surprised me was scaling of textures after using octane .ocs exporter. I have some geometry exported using TIG's plugin and some by Octane. These work in different ways. TIG's exporter stores all information of texture scaling in obj and mtl - and this is IMO the correct way of exporting geometry, while Octane exports scaling values into an image texture node in the UV input. See the screenshot. So when you change the texture the scaling is not valid anymore.
So my next wish is to unify the ocs scene exporter with TIG's exporter - and make it working like TIG's one.