The OpenQASM standard is only specified with gates, CX and U. All the rest of the gates we have worked with are user defined, within the IBM QX environment, available with the inclusion of qelib1.inc. IBM QX does not allow the direct access of these primitive gates.
If you look at the content of the qelib1.inc file (https://github.com/Qiskit/openqasm/blob/master/examples/generic/qelib1.inc), you can see how these definitions are made. In the following sections, we will go over the primitive CX and U gates to give you enough understanding.