In your first post, you showed this rule:
Is that a rule in the BO Jobs? That won't work to adjust what is listed in the combobox. Instead, for the attribute om_JobAreas in Jobs (which I assume has a Combobox as its presentation widget ), you need to set the Record sorting/filtering.
After clicking the ellipsis for this property, you can tick the box "Filter records" and then specify which records you want included. Either by way of an existing query or you can define a custom search query just for this purpose. The suggestion given by Bruce "FIND JobAreas where JobAreas.ob_Jobs = Jobs" seems right to me for this search query. To avoid confusion, I wouldn't call it a rule. It's a query (which can be written in Textual view, but that doesn't make it a rule in my opinion).
As a side note: your rule "Find JobAreas WHERE (JobAreas IN Jobs.om_JobAreas) | Jobs.om_JobAreas = JobAreas " won't do anything. The fist line finds all JobAreas that are in this specific Job. The second line puts those JobAreas into the same Job. But they were already in the Job, that's how you got them in the first place. So nothing happens.
Furthermore, it's good practice to use single rather than plural for BO names. So use Job instead of Jobs and JobArea instead of JobAreas. That makes it more intuitive in your rule that you refer to a single object rather than multiple. For the attribute in the parent BO that refers to multiple children, you could use the plural form to indicate that there may be more than one (i.e. Job.om_JobAreas).
