modo and Octane take different approaches to creating smooth surfaces. In modo you can use sub-D to get the software to interpolate a smooth surface which shows up correctly in the modeling windows and in the render. In Octane you are dealing with the actual polygons you export (which can be more or less smooth, but there's a switch on the materials (last setting at the bottom of the list) to set them to smooth: this will usually give you the effect you want.
If you want to be really accurate, subdivide to create a very close approximation to what you want in modo, and export that to Octane, then use material smoothing as the final step.
I've had good results with that same couch (I think) from the modo content library.
