At time of deployment, you have your choice of OS for the host machines on which Domino is installed. Domino currently supports Ubuntu 16.04, CentOS 7, and RHEL 7.3+.
It's worth noting that we offer Domino-managed standard Docker images for compute environments in Domino. The full versions of these standard images are Ubuntu only at the moment, but we do have a minimal base image for CentOS, and this should work for RHEL also.
Please contact us or comment below if you are interested in using RHEL and have further questions.