OCI is the Oracle Call Interface, used for direct programming access to an Oracle Database. Similar to ODBC, it is specific to Oracle databases.
The Oracle Call Interface (OCI) is a C language programmatic interface specific to Oracle databases. The API is database version agnostic, meaning programs written using the OCI API against Oracle version 8, will continue to function correctly against version 11gR2.
It is used over generic interfaces like ODCB because it allows access to Oracle database specific features like connection pooling, security access, and data types.