What you describe is called cascading (or dependent) comboboxes. This requires VBA behind a form. Won't work with table. I NEVER build lookups in table, just comboboxes on form. This is a very common topic.
So, with my table design, UnitID combobox on form would have a RowSource like: SELECT OrgUnitID, OfficeName FROM OrganizationUnits WHERE LevelID = [cbxLevelID];
VBA in event procedure(s) would requery the combobox: Me.cbxOrgUnit.Requery
Some prefer to set RowSource property SQL statement instead of just requerying combobox.
The trick is figuring out what event(s) to put code into - usually form Current and combobox AfterUpdate and/or GotFocus.
Unfortunately, this will probably mean a field in Employees for LevelID. Alternatively, use form/subform arrangement. Main form bound to OrganizationUnits and subform bound to Employees. Master/Parent Links set to the associated OrgUnitID and UnitID fields. UnitID will automatically populate with OrgUnitID value when creating employee record.
Be aware cascading combobox with lookup alias doesn't work nice with continuous or datasheet form.
You already have a 'picture' of suggested design in my previous post. And it looks like from your other thread you have adopted that structure. Consider this thread resolved.