Opportunity Product form, where is the Opportunity Lookup field?

Hi guys,

I'm trying to modify something on the Opportunity Product form. I want to pre-filter to existing product on this form to show only products of a certain categorie. The categories (custom) are selected on the Opportunity form. I see there is a field on the Opportunity Product entity that is a Lookup field to the Opportunity, but I can't add this field on the Opportunity Product form?

Why is this and how can I retrieve the data from the opportunity from within the Opportunity Product form?

Grts,

Roel