OneAPI builds on top of SYSCL, is basically Intel's CUDA, which it is already the second attempt to have C++ in OpenCL, during OpenCL 2.x, an effort that worked so well, that OpenCL 3.0 is basically a reboot back to OpenCL 1.0.
Also even SYSCL only got a proper kick-off after CodePlay came up with its implementation, nowadays they sell oneAPI support and tooling, after being acquired by Intel.