Oracle C++ Call Interface (OCCI) is a high-performance and comprehensive API to access the Oracle database.

From the Oracle product page:

Oracle C++ Call Interface (OCCI) is a high-performance and comprehensive API to access the Oracle database. Based on Standard C++ and object-oriented paradigm, OCCI is designed for improved productivity and quality in developing Oracle database applications.

Introduced in Oracle9i, OCCI is being successfully used for client-server, middle-tier, and complex object modeling applications.