I am trying to embed an editable table containing child records to a parent table that has a conditional proxy field. Help!

My Parent table, "Release Orders" has many "RO Line Items."  I have another relationship where "Line Items" have many "RO Line Items." When the user adds an RO Line Item from the Release Order, their Proxy Field, "Select a Line Item" is a conditional lookup field - where the Line Item OA# must equal the RO OA Number. My users do not want to add line items where they have to save each time.

I want to embed a grid editable table within my Release Order Table that allows users to select their Line Item quickly - but I am struggling with the condition of first having to choose the OA Number. There has to be an easier way...