Same exact issue here since we’re using some op_query as well with mongoose version 5.X.X. Are you able to solve the problem? I tried to upgrade to mongoose 6.x.x but too many errors to deal with.
It would be best to create a new post with the exact details/errors. However, in saying so, there were some email(s) sent out several months ago advising to test the new version and included some details on what to do with regards to the upgrade. Refer to also the relevant change notes for 6.0 and Legacy Opcodes.