OBP = OnBase Percentage
SLG = Slugging Average
MLB OBP = MLB Average OBP (with pitchers removed)
MLB SLG = MLB Average SLG (with pitchers removed)
BFP = Ballpark Factor
The formula is
OPS+ = (OBP/MLB OBP + SLG/MLB SLG  1) x 100/BPF
Using Miguel Cabrera's 2012 season as an example yields
OPS+ = ((.393/.324 + .606/.413  1) x 100)/ 102 = 165
In general, an OPS+ of 100 is average, an OPS+ of above 100 is above average and an OPS+ of less than 100 is below average. There is a popular misconception that OPS closely matches the ratio of a player's OPS to league OPS. However, an OPS+ of 165 does not mean that Cabrera had an OPS 65% better than league average. We know it's a really high OPS+ because it was the highest in the American League, but it has no concrete meaning.
Another limitation of OPS+ is that it counts OBP and SLG the same when OBP actually contributes about 80% more to run scoring than SLG. Thus, players who get most of their production from OBP will be short changed by both OPS and OPS+ . At any rate, the OPS+ figures can be found at BaseballReference.com for all players.
The OPS+ metric is OK for many purposes as long as you understand the shortcomings. If you want a more reliable statistic, you can use Weighted Runs Created Plus (wRC+), a creation of Tom Tango. Because it is based on Weighted OnBase Average (wOBA), wRC+ more accurately weights batting events (1B, 2B, 3B, HR, BB, HBP, outs) than OPS does.
It is calculated as:
a = MLB Runs per PA ( with pitchers removed)
b = Park Adjusted wRAA/PA (with pitchers removed)
c = b/a + 1
wRC+ =c x 100
Cabrera had the following numbers in 2012:
a = .1164
b = 53.7/697 = .0770
c= .0770/.1164 + 1 = 1.66
wRC = 1.66 x 100 = 166.
Another benefit of wRC+ beyond it's accuracy is that it has a concrete interpretation. In Cabrera's case, he created 66% more runs than would be expected by an average hitter in 697 PA. The numbers for all players can be found at FanGraphs.com.
The wRC+ metric is on the same scale as OPS+ and does not generally produce wildly different results. The biggest difference I found for the Tigers was Austin Jackson (135 wRC+ versus 130 OPS+). The OPS+ vs. wRC+ comparison for the rest of the Tigers in 2012 is shown in Table 1 below. The third colum is the percentile among players with 250 or more PAs.
The lesson to be learned here is that wRC+ is a little better than OPS+ and should be used for more serious evaluation of players. However, if you prefer using the BaseballReference site, OPS+ is a reasonably good estimate of a player's relative hitting value in most cases.
Table 1: wRC+ versus OPS+ for Tigers, 2012
Player

OPS+

wRC+

PCTL

Cabrera

165

166

99

Fielder

152

153

97

Jackson

130

135

88

Dirks

130

133

87

Hunter (LAA)

132

130

85

Avila

100

104

52

Infante (DET/MIA)

93

92

34

Berry

86

89

29

Young

89

89

29

Peralta

85

86

24

Boesch

77

77

13

Santiago

52

55

1

Raburn

30

28

0
