How can I extract only those opportunities with fewer than 5 activities in BI?

You can use subject areas provided by Oracle to create the Oracle Transactional Business Intelligence (OTBI) reports.

Here's a simple example:

SELECT
"Sales - CRM Sales Activity"."Opportunity"."Opportunity ID" s_1,
"Sales - CRM Sales Activity"."Opportunity"."Opportunity Name" s_2,
"Sales - CRM Sales Activity"."Activity Facts"."# of Activities" s_3
FROM "Sales - CRM Sales Activity"
WHERE
("Activity Facts"."# of Activities" < 5)

The subject areas respect data security. The person running the report sees only the date they have permission to view.