The Ovenized Crystal Oscillator, or OCXO, is a highly temperature stable frequency oscillator. It uses a crystal oscillator (XO) that is housed inside of a shielded and insulated compartment which is then heated to a high temperature. By controlling the temperature inside of the compartment, the oscillator will not drift when exposed to different (exterior) ambient temperatures, as long as those temperatures are below that inside the insulated compartment.
Some OCXO devices have options for frequency adjustment, making them an ovenized voltage-controlled crystal oscillator, (VCXO). An OCXO offers better frequency stability than any of the XO, VCXO, or TCXO options.