The ORB is the average of the past 10 days smallest range from the open to the high or the open to the low. The idea is once this has been broken, then you're more likely to be trading the primary trend of the day. This is just the basic original calculation. Toby went into great depth on all the nuances like the difference between early breakout versus late breakout, reversals because of weak penetration of the orb, using patterns of days leading up to today to improve the results, stops, using limit orders to capture the early range (before the first retracement), etc. I posted the results of using it from 1/1/96 - 6/30/2001 under the Opening Range Breakout thread under Resouces if you're wondering how it performed without anything added. This is all considered out of sample testing since the idea first came out in 1989. If you want to work on early entry ideas, this is a good place to start.