OctaneRender® V4 requires an Nvidia driver supporting at least CUDA 9.1. On Windows and Linux, a recent enough Nvidia driver (http://www.nvidia.com/download/index.aspx?) is necessary.
For Mac OS X and macOS versions, use the most suitable driver released from the Nvidia website (http://www.nvidia.com/object/mac-driver-archive.html).
Failure to install these driver versions may result in stability problems. The OctaneRender Team cannot provide support to users of different driver versions.
The CUDA driver is the part of the NVIDIA driver stack to which Octane uses. On Linux and Windows it is part of the NVIDIA graphics driver, while on MAC systems it is a separate installation.
If you install any recent Nvidia graphics driver on Linux and Windows, it will install a CUDA driver that supports CUDA 9.1. The CUDA Toolkit is only for development and does not have to be installed. The Toolkit does include a graphic driver, however, it may not be the latest.