Hi,
never did it with OpenCart but I built several php command line scripts to auto-generate price and stock feeds for Prestashop sites.
The solutuon would be a php script that will run as cronjob at scheduled intervals, connect to the store database, fetch each product and write price and stock to an xml feed.
I assume from there your price comparison partner will parse it and update its database.
My price is fixed. message me if interested.
Regards,
Calin.