Sunnyhope - I'm no expert at all on lenses, but do know the basics. In 'lens-speak' OS means "optical stabilisation".
If you're moving from midrange or bridge-zoom P&S cameras to DSLR, you'll have been used to having the "anti-shake" or "stabilising" built-in to the camera itself.
Some DSLRs, including Pentax, do have the "SR", or shake-reduction, built into the camera body. Such DSLR bodies "stabilise" all lenses mounted on them, including the wide variety of Film-SLR lenses from past eras - this can be beneficial for people with low fixed incomes, like myself.
However, the most popular / widespread DSLRs, Canon and Nikon, don't put the stabilising in the camera body. That means they can then offer ranges of lenses without or with stabilising in the the lenses - those without it can have the same optics (not always) - but be lower cost than the lenses with stabilising added.
There's an ongoing "debate" about whether in-body or in-lens stabilising is best - but from Forums and Reviews the Nikon / Canon in-lens stabilising being best seems to be ahead - particularly for "long and fast'' lenses used for rapid action such as Sports.
There's a Review of the lens you mention at:
- http://www.photozone.de/Reviews/321-sig ... rt--review