I just got a Sram cassette and notice that the first few small cogs is missing a tooth (i.e. 12has 11 tooth, the 13 has 12 tooth). is it how it's suppose to be??? I enlarge the picture at performancebike and it seems like it is that way. Can someone please tell me before I install it??? Thanks!!!!

Yes. Read the directions that came with it.

OG = Open Glide. Short version is yep, that is correct. Supposed to make shifting easier and smoother.

Zing!

And, yeah, RTFM n00b. :)

moreover, the new red cassette has open glide all the way up the cassette, and has up to three missing cogs off the top gear.