Batting average, for example, doesn't give the batter credit for drawing a walk. OPS also rewards a batter's ability to get hits, especially extra-base hits, by including slugging percentage as a component. A double typically is more valuable to a team than a single, and this is reflected in the formula. Home runs are the most valuable type of hit, and the formula gives them the most weight.
http://www.wisegeek.org/in-baseball-what-is-ops.htm
The theoretical max for Slugging is 4 (assuming you get a home run at every at bat) to determine OPS; you just add the on-base percent to the slugging. This makes the theoretical max for OPS: 5.000, unlike other stats where 1.000 is generally the max. This is simply on-base percent plus Slugging. It is a stat similar to Gross Production Average -- it gives a number.
http://www.answers.com/Q/How_do_you_calculate_OPS_in_baseball
Another criticism of OPS is based on the formula for slugging percentage. According to the formula, a double is worth twice as much as a single, a triple is worth three times as much, and so on. In terms of value to the team, however, the difference between the types of hits has been found to be much less.
http://www.wisegeek.org/in-baseball-what-is-ops.htm
A player's on-base percentage is the sum of his or her hits (H); walks, or bases on balls (BB); and times hit by a pitch (HBP) divided by the sum of his or her official at-bats (AB), walks, sacrifice flies (SF) and times hit by a pitch.
http://www.wisegeek.org/in-baseball-what-is-ops.htm
A baseball statistic that is calculated by determining the total number of bases a batter has reached (home runs are counted as four) divided by their total number of at-bats. SLG = (1B + 2 x 2B + 3 x 3B + 4 x HRs)/At-bats.
http://www.sportingcharts.com/dictionary/mlb/slugging-percentage-slg.aspx
Sporting Charts explains Slugging Percentage - SLG. Slugging Percentage (SLG), like On-base Plus Slugging Percentage (OPS) is one of the statistics that helps define a batter who hits for power.. Typically players who hit for power have a greater number of extra-base hits and will therefore have a higher Slugging Percentage. Slugging Percentage Records.
http://www.sportingcharts.com/dictionary/mlb/slugging-percentage-slg.aspx
1 For a single fielded by the RF or CF, the # of times the runner was held at second (and there was not a runner on second held at third). 2  For a single fielded by the RF or CF, the # of times the runner advanced to third.  For a single fielded by the RF or CF, the # of times the runner was out on the bases.
http://www.baseball-reference.com/about/war_explained_position.shtml
In order to be among the league leaders in OPS, a playe…r must hit for average, display a great batting eye (to collect walks), and hit for power. An OPS over 900 is considered quite good, and an OPS over 1000 is sure be among the league leaders.
http://www.answers.com/Q/How_do_you_calculate_OPS_in_baseball
1 For a single fielded by the RF or CF, the # of times the runner was held at second (and there was not a runner on second held at third).  For a single fielded by the RF or CF, the # of times the runner advanced to third.
http://www.baseball-reference.com/about/war_explained_position.shtml
On-base percentage plus slugging percentage (OPS), also called on-base plus slugging, is a statistic that attempts to measure a hitter's performance in baseball.
http://www.wisegeek.org/in-baseball-what-is-ops.htm
how to calculate ops baseball
To calculate On-base plus slugging baseball, add the on-base percent to the slugging. To calculate the on-base plus slugging baseball add the on base percent to the slugging.