It looks like you have the "resolution gate" problem.
I had this problem many times. When you set your setting to anything else than "fill", octane does not match the settings.
I've tried evry settings, the only one that works is "resolution gate : fill".
p3taoctane
This was reworked to more properly reflect the resolution gate of rendered image in Octane. Just enable the "Resolution gate" mode in viewport camera settings and you will see the exact fit.