A practical starting point
A passive crystal works with an oscillator circuit. An oscillator integrates circuitry and provides a clock output when powered. Start with the host IC clock-interface requirements.
Different devices, different checks
For crystals, review load capacitance, ESR, drive level, initial tolerance and temperature conditions. For oscillators, check supply, output levels, loading, enable and start-up. Both require exact frequency, package and footprint review.
Keep functions separate
XO, TCXO, VCXO and OCXO describe different functions. TCXO concerns temperature compensation; VCXO concerns voltage tuning; OCXO uses oven control. Check the complete part specification for combined functions.
When preparing an inquiry
Provide the host IC, existing part or datasheet, frequency, package, temperature range and quantity. For alternatives, distinguish fixed requirements from negotiable ones.