The Oban cron jobs installed modules declare, so a host does not have to copy them into its config by hand.

A module declares its scheduled workers with the optional PhoenixKit.Module.oban_crons/0 callback, next to PhoenixKit.Module.oban_queues/0:

@impl PhoenixKit.Module
def oban_crons, do: [{"0 3 * * *", MyModule.Workers.NightlyRebuild}]

mix phoenix_kit.install writes every declared entry into the new crontab and mix phoenix_kit.update adds the ones an existing crontab is missing (PhoenixKit.Install.ObanConfig.ensure_module_cron_entries/3). The host's say wins, as it does for core's own workers: a schedule the host wrote is never changed, and an entry the host commented out of its crontab is a declined entry that is not offered again.

Entries are read from every installed module, enabled or not, and a module is found without the registry when the installer runs before it exists (the same rule as PhoenixKit.ObanQueues). A worker should do nothing while its module is switched off.

One entry per worker

The installer decides whether an entry is already there by the worker's name, so a worker has one schedule and no args. If two modules declare the same worker with different schedules, the first (modules sorted by name) wins and the other is reported as a conflict.

What is dropped

An entry is ignored, with a warning, when the cron expression is not one Oban's own parser accepts (Oban.Cron.parse/1, or Oban.Plugins.Cron.parse/1 before Oban 2.24 renamed the plugin) or the worker is not a loaded Elixir module exporting perform/1. That is deliberate: Oban refuses to start when its crontab names something that is not a worker, so one bad declaration must not reach the host's config.