I've covered the detailed setup of the IDCS confidential app for OIC in a previous post. You can check it out here.
There are two things that seem to trip people up, when trying out the factory api from Postman etc.
1. The url - must be in the format - https://design.integration.region.ocp.oraclecloud.com/factoryApi?integrationInstance=yourOICInstanceName
So let's try out the factory api to retrieve OIC Projects -
Looks good, now let's try the following factory api -