Ok, this seems a limitation of the Octane Material Converter:
XFrog uses the same map both for diffuse and opacity, using the RGB and Alpha channels respectively (according the behaviour of Bitmap Parameters->Mono Channel Output). But Octane ignores this property, and use the RGB channels directly as mask texture.
XFrog includes separate b/w TIFs for opacity maps, that can be used for the opacity slot. Unfortunately, replacing them is a manual process.