Office communication system
Oil level indicator
Omnidirectional ultrasonic intrusion surveillance unit
Operation state monitoring apparatus
Operator alerting device
Optical fire detection system
Optical laser security system
Order communication system for restaurant
Order communication system for restaurant
Orientation responsive alarm system
Oscillator energized liquid level indicator
Output circuit of an ionization smoke sensor
Overload protection device
Overload safety device for telescopic cranes
Overtemperature monitor and integrator apparatus