O
off-ramp gateway	In the Store-and-Forward Fax application, a gateway that provides an exit from the packet network to the PSTN or nonpacket device.
OGK	Originating gatekeeper.
OGW	Originating gateway. Voice gateway that handles calls entering the IP network.
OID	Object identifier. Values are defined in specific MIB modules. The Event MIB allows a user or an NMS to watch over specified objects and to set event triggers based on existence, threshold, and boolean tests. An event occurs when a trigger is fired; this means that a specified test on an object returns a value of true. To create a trigger, a user or an NMS configures a trigger entry in a table in the Event MIB that specifies the OID of the object to be watched. For each entry type, corresponding tables (existence, threshold, and boolean tables) are populated with the information required to carry out the test. A MIB can be configured so that, when triggers are activated (fired), either an SNMP set is performed, a notification is sent out to the interested host, or both. See also MIB.
OIR	Online insertion and removal. Feature that permits the addition, replacement, or removal of cards without interrupting the system power, entering console commands, or causing other software or interfaces to shut down.
on-ramp gateway	In the Store-and-Forward Fax application, a gateway that provides an entrance to the packet network from the PSTN or a nonpacket device.
originator	User agent that initiates a Transfer Request or Refer Request message with the recipient.
OSI reference model	Open Systems Interconnection reference model. Seven-layer reference model and framework of standards for communication in a multisystem environment.
OSP	Open Settlement Protocol. Client/server protocol for establishing authenticated connections between gateways and for allowing gateways and servers to transfer accounting and routing information securely. Allows service providers to roll out VoIP services without establishing direct peering agreements with other ITSPs.
OSPF	Open shortest path first. Link-state, hierarchical routing algorithm. Features include least-cost routing, multipath routing, and load balancing.
OSS	Operations support system.
overlay	Placement of up to 10 ephone-dns on a single phone button. An overlay set is a group of ephone-dns that are overlaid onto a single phone button.