The Object Manager cache should be at least as large as the sum of the number of tables, indexes, and LOBs in the database.
Determine what the -omsize should be by running:
select count(*) from _storageobject.
If that is inconvenient, open the Configuration dashboard in the ProTop Portal and up the numbers in the red boxes below (bottom center of the Configuration dashboard page):
If the total plus 10% is lower than 1024, it is unnecessary to change it. If it is higher (as it is here), set -omsize to your total plus 10% in your database startup parameter file and restart the database.