Clocks and oscillators
OCXO
Also: oven-controlled crystal oscillator
An OCXO keeps the quartz crystal at a constant, regulated temperature (the 'oven'), which removes the largest source of drift. It is considerably more stable than a TCXO and delivers much better holdover.
The trade-off is higher power consumption and a warm-up time. Widely used in master clocks and grandmasters.