Hi Aki,
The pressure an O-ring is good for is dependent on something called the "extrusion gap". That's the gap between two parts on the low pressure side of the seal and into which the O-ring is forced (extruded) by the pressure. This gap distance is a function of the O-ring's hardness. The harder the material, the larger the extrusion gap can be. Here's a chart that gives that information:
http://www.marcorubber.com/msg-fig6.htm
(They call extrusion gap the "clearance gap" on this web page. And the hardness of the O-ring increases as you go from 60 to 90 shore A.)
If pressure is too high, a backup ring can be used. I've not seen any good sources for this, but I've seen O-rings used up to extremely high pressure with backup rings or with essentially zero extrusion gap.
Regarding materials, Parker is debatably the leading O-ring manufacturer in the world. They have a material compatibility chart here:
http://www.parker.com/o-ring/fcg/fcg.asp
Parker also has a designer handbook available on the net here:
http://www.parker.com/o-ring/Literature/ORD5700.pdf
Visit their website for O-rings here:
http://www.parker.com/ead/cm2.asp?cmid=3106
(they make a vast variety of hydraulic and pneumatic equipment, so you may suddenly find yourself on a different division's web site.)
Take a look on the third line down (black first line, grey second line, white third line) where it has the headings:
O-ring
Products> Services> Literature> Markets> Applications>
And click on Literature which opens up a drop down menu. Each listing takes you to another web page with a bunch of different information on O-rings.
If you have specific questions, I'll try and answer them. As Gokul has suggested, you need to be specific. Pressure and temperature is needed, along with what fluid you're sealing, and if it's a static or dynamic application, a rough sketch of what you're trying to do would help. We might then be able to steer you toward a gland design that would fit your application.