Note: Older versions of Octane have Shadow Catcher named as "Matte" insted. The function is the same, just with an old name.

3 Ways to create an infinite floor effect

#1, use Shadow Catcher under Common on a diffuse material applied to the floor of your scene.

#2 Simply use a gradient on the floor of your scene to smooth out the edges of it to make it blend in and look seamless. Make sure your floor color matches the background.
This is somewhat of a hack, and have limited functionality, like not working with an alpha for your background if you wanted to composite later.

#3 Enable Render Passes to then turn on Render layer and set your layer ID to 1 and check on black shadows and reflections, then put an octane object tag on your floor and set its layer ID to 2. Example image.

You can enable the alpha channel in settings to get a background that is transparent. This is great for compositing later on.