Hi,
I am not sure if I understood your requirement completely, but you said that you are exporting OSPF routes to internet peers using BGP. So I guess you must be using an export policy to redistribute OSPF routes into BGP? If that is the case, you can use metric or as-path-prepend in BGP to achieve the required preference changes.
You can refer to following links for more details:
Configuring a Routing Policy to Prepend the AS Path
Thanks