All DLLs inside the OptiX bin64 folder are redistributables. Ship the ones you need with your application.
Similar for Visual Studio libraries. The MSVS comes with a folder of release redistributables, for example in C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\redist\x64.
(Note that debug libraries are non-redistributables!)
Or the end user would need to install the release redistributables (vc_redist.x64.exe) on the target machine.
Find more info about that here: https://docs.microsoft.com/en-us/cpp/ide/determining-which-dlls-to-redistribute?view=vs-2017
End users do not need to install the OptiX SDK or any CUDA toolkit to run OptiX applications.
All they need is a supported GPU and driver combination.
The error “Failed to load Optix library” is most likely due to an unsupported GPU - driver combination.
OptiX 6.0.0 requires a Maxwell or higher GPU and at least a 418.81 display driver under Windows and 418.30 or higher under Linux.
Read the OptiX release notes on the OptiX download page again.
Note that when shipping commercial applications, the NVIDIA license you agreed to when downloading OptiX requires, that you inform NVIDIA about that by filling out a short notification page. The link is inside that license document.