When selecting an O-ring, the following factors need to be considered:

1. Material selection: The O-ring can be made of various materials, such as rubber, silicone, fluororubber, etc. Choosing suitable materials should consider the temperature range of the working environment, chemical resistance, pressure requirements, and special application requirements.

2. Size and geometric shape: Select the appropriate inner diameter, outer diameter, and cross-sectional thickness according to the application requirements to ensure that the O-ring can be properly sealed and adapted to the required size.

3. Working conditions: Consider the working environment in which the O-ring is located, including temperature, pressure, chemical properties of the medium, and motion conditions (such as static or dynamic sealing). These factors will affect the material selection and performance requirements of the O-ring.

4. Durability and lifespan: Select O-rings that meet the durability and lifespan requirements according to the application requirements. This includes considering the wear resistance, aging resistance, corrosion resistance, and shear resistance of the O-ring.

5. Standard compliance: Ensure that the selected O-ring complies with relevant standards and specifications, such as ISO 3601, ASTM D2000, etc., to ensure its reliable quality and performance.

6. Cost effectiveness: After considering the above factors comprehensively, choose an O-ring that meets the requirements, and seek cost-effective options as much as possible while meeting performance requirements.

Taking into account the above factors, an O-ring suitable for a specific application can be selected to ensure its effective sealing and meet the required performance requirements.