The RHEL 8.0 inMemory build variant uses rhel80-medium machines for its 'large' tests. These machines have 16GB of memory available, but this is not enough for some workloads.
For example, the concurrency_replication_causal_consistency workload uses 5 mongod nodes, and by default, we give each inMemory node 4GB of WT cache. Together, these nodes are allocated 4GB more memory than is available in the system, and that's just for the WT cache.
Increase the "large_distro_name" to a larger machine to avoid OOMs.