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Occupancy
Zoning and safety code term used to determine how a structure is permitted to be used and occupied, which in turn dictates the necessary safety structures and procedures.


Occupancy class
General categories of structures for purpose of safety planning, such as for hospital, assembly, industrial, single-family dwelling, apartment building, commercial, etc. Further broken down by types of hazards associated with particular occupancies, such as gas stations.


Occupant use hose
Light-weight firehose coupled to standpipe for emergency use by building occupants prior to arrival of firefighters. Often accessible by breaking glass to unlock secure enclosure.


Offensive attack
Method of firefighting in which water or other extinguisher is taken directly to the seat of the fire, as opposed to being pumped in that general direction from a safe distance.


On-call
Personnel who can be summoned (and paid) when necessary to respond to an incident; a type of volunteer" fire department.


One-hour fuel
Vegetation with large surface-to-mass ratio, a so-called fine fuel" (along with 10-hour) that quickly reaches critical (inflammable) moisture levels (fine fuel moisture FFM) when exposed to heat; compare with 100-hour or 1000-hour fuels (i.e. live fuel moisture LFM) which take much more heat to ignite.


Open-circuit SCBA
See SCBA. Exhaled air is not reused by the system.


OSHA
U.S. government agency concerned with regulating employee safety, particularly in hazardous occupations such as firefighting.


Outside fire
Urban fire not inside a building or vehicle, often found to be burning trash which could extend to nearby structures or vehicles if not dealt with properly. A suburban, interface, or rural outside fire could also be a wildland fire.


Outside stem and yoke valve (OS&Y)
Type of gate valve actuator arranged such that the valve stem moves in and out of the handle, thus externally indicating whether the valve is open or shut, unlike the more common gate valve wherein the stem rotates and only the gate moves up and down inside the fixture.


Overhauling
Late stage in fire-suppression process during which the burned area is carefully examined for remaining sources of heat that may re-kindle the fire. Often coincides with salvage operations to prevent further loss to structure or its contents, as well as fire-cause determination and preservation of evidence.


Overhead
Personnel assigned to supervisory positions, including Incident Commander, Command Staff, General Staff, Branch Directors, Supervisors, Unit Leaders, Managers, and staff.


Oxidizer
A hazardous material containing oxygen that can combine with adjacent fuel to start or feed a fire.