Operation and maintenance documentation organized by system, with the installed equipment's model, serial, parts, and maintenance dataIn: Punch list and closeout
The owner-architect-contractor meeting, the formal progress and decision forum, usually weekly or biweekly during active constructionIn: Progress meetings field guide
Normal neutral current flowing on the equipment grounding system, caused by more than one neutral-to-ground bondIn: Grounding vs bonding explained
Obscuration per foot
How much smoke dims light over a distance, the unit the detector's sensitivity and alarm thresholds are set inIn: Aspirating smoke detection
Observability
Seeing what the network is actually doing in detail, enough to answer why a job was slow, not just whether links are upIn: Network observability for the AI fabric
Optical-electrical-optical conversion, turning light into an electrical signal and back; what an OCS skips and a packet switch must doIn: Optical circuit switch and photonics
Shafts parallel but off-center (offset) or meeting at an angle (angular); both are corrected with dial or laser alignmentIn: Hydronic pump install field guide
The heat buyer or user who takes the recovered heat, ideally nearby with year-round demandIn: Data center waste heat reuse
OGFC
Open-graded friction course, a porous surface mix that drains water and cuts spray and noiseIn: Asphalt mix types by layer
OH&P
Overhead and profit, the markup on a change; entitlement is recognized but the percentage is commonly set by the contractIn: Field change order takeoff
Ohm
The unit of electrical resistance; ESD floor values run high and are written in scientific notationIn: ESD floor testing field guide
Ohm and scientific notation
The resistance unit; ESD values run high, so 1.0 x 10^9 ohms is one gigohm and 1.0 x 10^6 ohms is one megohmIn: ESD floor testing field guide
Oil analysis
The blood test of a machine: wear metals, contamination, and lubricant condition from an oil sampleIn: Predictive maintenance guide
Oil canning
Cosmetic waviness in the flat of a panel, not a structural defect or a leakIn: Standing seam metal roof
Oil logging
Oil that will not return collecting in the evaporator, filming the tubes and cutting capacityIn: Oil return field guide
Oil return
Keeping the compressor's oil circulating with the refrigerant and back to the crankcaseIn: Oil return field guide
Oil separator
A discharge-line device that returns most oil to the crankcase near the sourceIn: Oil return field guide
Oil trap (P-trap)
A U at the base of a riser that pools oil and slugs it up; inverted at the top to stop drain-backIn: Oil return field guide
The minimum vapor speed that drags oil back to the compressor; the controlling number on suction and hot-gas risers at minimum loadIn: Refrigerant line sizing field guide
Oil/water separator
Tank that floats petroleum oil and settles grit out of wastewater before discharge; also oil separator or oil interceptorIn: Oil/water separator field guide
The early solid aluminum branch-circuit wire of roughly 1965 to 1973, the material this guide is aboutIn: Aluminum branch wiring
OLTS
Optical loss test set, the light source and power meter pair used for Tier 1In: Fiber OTDR certification
OLTS / OTDR
Optical loss test set for Tier 1 end-to-end loss; optical time domain reflectometer for Tier 2 event-by-event tracesIn: Data center structured cabling
OM3 / OM4 / OM5
Laser-optimized multimode grades; OM3 and OM4 aqua at 850 nm, OM5 lime green wideband for SWDMIn: Data center fiber types
OM4 / OS2
OM4 is laser-optimized multimode for short reach; OS2 is low-water-peak singlemode for backbone and long high-speed runsIn: Data center structured cabling
The distance from the center of one plant to the center of the next, which sets the plant count per square footIn: Seasonal color and annual rotation
On-site / behind-the-meter (BTM)
Power generated at the data center, behind the utility meter, instead of bought from the gridIn: Data center on-site generation
On-the-way text
An automated message sent when the crew is dispatched, with an arrival time and the technician's name, for trust and safety before the visitIn: Customer communication and reviews
Operating expenditure: the recurring cost to run, dominated by energy, then maintenance, staff, water, and leaseIn: Data center TCO cost model
OPIM
Other potentially infectious material, the body fluids and tissues beyond blood that the bloodborne pathogens standard also coversIn: Biohazard and trauma cleanup guide
OPR
Owner's project requirements, the measurable statement of what the owner needs the building to do, the yardstick for every testIn: Data center commissioning process
An active mitigation system that trips on the light of an arc confirmed by current, clearing in millisecondsIn: Arc energy reduction methods
Optical circuit switch (OCS)
A switch that steers light from an input fiber to an output fiber as a held path, with no optical-electrical-optical conversion or packet processingIn: Optical circuit switch and photonics
The contract's stated hierarchy of which document governs when two documents conflictIn: Document control field guide
Organic matter
Percent of decomposed material in the soil; drives water and nutrient holding, commonly 3 to 6 percent in good turf soilIn: Turf fertilization and soil testing
Organic matter (OM)
Decomposed plant and animal material in the soil, reported as a percentage; 2 percent or more is healthyIn: Soil preparation and amendment
Single-mode grades; OS2 is low-water-peak glass for long and outside-plant runsIn: Data center fiber types
OSDP
Open Supervised Device Protocol, the SIA standard for encrypted, supervised, two-way reader-to-controller communication, replacing legacy WiegandIn: Access control system field guide
OSHA 1926.1153 / PEL
The construction silica standard; PEL is 50 micrograms per cubic meter as an 8-hour TWA, with a 25 microgram action levelIn: Silica exposure control program
OSHA 1926.62
OSHA's lead in construction standard governing worker exposure, with a 50 ug/m3 PEL and a 30 ug/m3 action level as 8-hour averagesIn: Lead-safe renovation (RRP)
OSHA 300 / 300A / 301
The recordable injury log, the annual summary, and the incident report under 29 CFR 1904In: Construction safety program
OSHA Subpart R
29 CFR 1926 Subpart R, the steel erection standard covering anchorage, stability, connections, and fall protectionIn: Steel erection and connections
A dense, weather-tolerant surface such as porcelain slab, granite, soapstone, or concrete; not laminate, marble, or engineered quartzIn: Outdoor kitchen construction guide
Outdoor kitchen
A built-in cooking and prep cabinet outdoors, combining structure, gas, electrical, plumbing, counters, and appliances in the weatherIn: Outdoor kitchen construction guide
A grill, refrigerator, or burner listed for outdoor built-in use, with corrosion-resistant materials and sealed components; indoor units are notIn: Outdoor kitchen construction guide
Outlet control structure
The riser, orifice, and weir assembly that sets the release rate, often multi-stage for different design stormsIn: Detention and retention ponds
Outrigger beam / counterweight / tieback
The cantilevered roof beam, the rated weights that hold its inboard end down, and the separate line tying it back to a sound anchorIn: Suspended scaffold and swing stage safety
Outriggers / cribbing
The extending legs and the bearing pads under them that keep the pump truck from tippingIn: Concrete pumping and placement
Outriggers / stabilizers
Legs that extend the base for stability; only effective fully set on firm bearingIn: Aerial lift and MEWP safety
Outside plant (OSP)
The outdoor part of the network, buried or aerial, from the central office out to homes and businessesIn: OSP fiber and FTTH construction
Collecting each job's share of overhead through the markup, so the cost of being in business gets paid across the year's work.In: Overhead recovery and bid markup
Overhead vs general conditions
Overhead is the whole-business indirect recovered through markup; general conditions are one job's indirect, recovered as job costIn: General conditions and indirect costs
Overlap (lap)
Running the tarp so the upper section covers the lower and the up-slope edge tucks under existing roofing, so water sheds over the top and never underIn: Emergency board-up and tarping
Overlay / resurfacing
A thin cementitious topping bonded over existing sound concrete to renew or decorate the surfaceIn: Concrete overlay and resurfacing
Device that senses running current and trips the contactor on a sustained overload before the windings overheatIn: Motor protection and overload relays
The independent mechanical brake, a parachute or block-stop, that clamps the mast if the platform descends too fastIn: Mast-climbing work platform safety
The fixed annual carry that runs whether the machine works: depreciation, cost of capital, taxes, insurance, storageIn: Equipment cost recovery and own vs rent
Owning and operating cost
Fixed owning cost plus variable operating cost, divided by hours worked, to get the cost per hourIn: Fleet maintenance program
Oxidizer
A gas like oxygen that does not burn itself but accelerates and intensifies the burning of other materialsIn: Compressed gas cylinder safety
PEX tubing with an EVOH layer that blocks oxygen diffusion, required on closed systems to protect ferrous parts from rustIn: Radiant floor heating design
Oxygen-fuel separation
Storing oxygen at least 20 ft from fuel gas or behind a 5 ft, half-hour fire-rated barrierIn: Compressed gas cylinder safety