Currently, by default, Ratis is not enabled on an OM single node cluster. This Jira proposes to change the default single node OM to use Ratis server.
OM Ratis has been tested extensively and is stable now. To convert a single node cluster to HA, the first step would be to upgrade the cluster to Ratis enabled cluster.
A non-ratis single node cluster can be upgraded to ratis-enabled single node cluster by just setting the ozone.om.ratis.enable config to true and restarting the OM.