This site gives info. on O-rings. The type of O-rings depend on what you are sealing and the pressure. If pressure is > 1500psi you will need back-up rings.
you can also find some valuable information. somewhere I must have a spreadsheet flying around with which you can determin the correct pre-load etc. its quite extended and can be used for o-ring selection very well. if you are interested I can look around where I saved it and send it.