Systems and methods are disclosed for providing online charging in an IMS network. An IMS gateway system receives a first message from a CSCF through a first interface for a call session, and processes the first message to determine whether to contact an OCS for online charging for the call session. If the determination is to contact the OCS, then the IMS gateway system generates a second message that comprises a charging request to transmit to the OCS. The IMS gateway system then maps fields of the first message in the first protocol to fields in the second message in a second protocol used by the OCS, and transmits the second message to the OCS through a second interface. The second message provides the OCS the proper information to perform online charging functions for the call session.