OCFS2 is a clustered filesystem. It's designed to be installed on a SAN storage LUN, and shared between systems that mount the LUN/filesystem. All nodes within the cluster (i.e. all nodes that mount the OCFS2 filesystem) can read and write to it simultaneously.
DRBD is block-level replication. It runs on top of block devices. Their project documentation (http://www.drbd.org/home/mirroring/
) mentions a primary-primary mode, presumably with DRBD replicating blocks, and OCFS2 handling the clustered filesystem locking. Sounds potentially complicated to troubleshoot. Your best bet is to search the 'net for tutorials, and test thoroughly in a test environment.
Local disk and SAN LUNs are both block devices. You replicate the data wherever you please.