OS market share figures were released again today, and though it is a rough indicator, it nonetheless is a consistent one that mirrors trends in the market. Back in July 2007, I noticed that Mac market share seemed to jump from August to January. After predicting that, it jumped from August 2007 to January 2008. Last July, I again predicted that Apple’s market share would jump again in the same period, this time guessing that it would reach up to 10%.

Guess what? Mac market share jumped from less than 8% to almost 10% between August and December 2008. Right on target. Click on the chart to enlarge it; the green areas denote growth periods:

Note also the new trend line in red (scale on right): the iPhone. Starting in June 2007 (with its release), it has been gaining market share rather significantly, very quickly–at this rate, in fact, it should overtake Linux in terms of browser market share (Linux currently has 0.85% of the market, the iPhone has jumped to 0.44% in 18 months).

If you combine the Mac OS and iPhone numbers, then Apple already has more than 10% market share.

What we’re seeing here in the general OS market is exactly what I predicted–the Mac share increases a few percent in the latter half of each year–and it’s not over. If the past is any indicator, then the rise should continue into January and perhaps February. In December, it was 9.63%; it still has one or two months of growth to reach the 10% mark.

Meanwhile, Windows continues its decline with 88.68%, and Vista continues it anemic climb, gaining slightly less than only 1% of market share per month (currently it’s at 21%, some 24 months after its release). Not good news for Microsoft.

But what’s really something is to extend a trendline for the Mac OS over time:

At this rate, the Mac OS could have a 25% market share by 2015, and about 45% by 2020.

Granted, that’s a pretty long-term extension of just a few year’s data. But keep in mind that I first mentioned this 14 months ago, and the new data falls only slightly short of the original trendline seen back then. Once again, hardly an iron-clad guarantee, but certainly a strong indicator that Apple has a great deal of growth potential. Certainly if you had said in 2002 that the Mac OS would have 10% market share by 2009, no one would have believed you. So, let’s wait and see.