OpenMP (Open Multi-Processing) is a "collection of compiler directives, library routines, and environment variables ... for parallelism in C, C++ and Fortran programs [1]."
While HIP can only take advantage of parallelism on GPUs, OpenMP is portable and can work on other hardware types such as CPUs. OpenMP also provides a way to easily add parallelism to an existing project with less effort than HIP. This does not mean that OpenMP is superior to HIP. HIP provides more control over what the GPU does and an optimized HIP kernel should out perform OpenMP.
If you already installed ROCm > 4.0 you should also already have OpenMP ROCm support. If not, install ROCm.
If you are using ROCm < 4.0 install openmp-extras.
Click here for more information on OpenMP support.
OpenMP can be used using compiler directives such as #pragma omp parallel.
Here is some OpenMP Tutorials and OpenMP Reference Guides.