Hello Doug,

To do this you need to change the domain of the product_id in the sale.order.line tree view on the sale.order form. Here is an example which only gets products where the product "default_code" is equal to the sales order's "client_order_ref". 

You would just need to change the field "default_code" to your field name on the product and change "client_order_ref" to the field on the sales order. 

You need to create an inherited view which can either be done using debug more or creating your own module and installing, code:

<xpath expr="//field[@name='order_line']/tree[1]/field[@name='product_id']" position="attributes">
 <attribute name="domain">
    [('default_code', '=', parent.client_order_ref), 
    ('sale_ok', '=', True), 
    '|', 
    ('company_id', '=', False),
    ('company_id', '=', parent.company_id)]
</attribute>
</xpath>

Also: you mention that yours is a selection field, if it is a Many2one you may need to add .id on the end of the right hand part of the expression. 

I hope this helps.

Thanks,