Don't worry, however, there is a solution. A stat called OPS+ is the solution, an OPS statistic adjusted to that player's ballpark and which league the player played in (American League or National League). The formula for OPS+ is:
OPS+ = 100 x (OBP/lgOBP*+SLG/lgSLG* - 1)
The easy way to understand lgOBP* and lgSLG* is that they are park-adjusted league-average on-base percentage and slugging-percentage. Basically, it's the average of how well the players played in those leagues in their ballparks. ESPN's Park Factor shows this equation for adjusted runs in a specific ballpark, thus creating lgOBP* and lgSLG*:
((home runs scored + home runs allowed)/(home games)) / ((road runs scored + road runs allowed)/(road games)
An OPS+ of 100 is exactly average, making it even easier to determine the value of a player. An OPS+ of 150 or greater is excellent, 125 is good, while an OPS+ of 75 is poor. Using this stat, you can determine how much better a player is than average (100 OPS+) or how much better a player was compared to another player of another era.
For example, if player A had an OPS+ of 110, and player B had an OPS+ of 80, player A can be considered to drive in 30% more runs than player B.
The only problems in OPS+ is that the stat weighs on-base percentage and slugging percentage equal to one another. Sabermetricians agree that OBP is more valuable than SLG, which somewhat throws the stat out the window. OPS+ is completely an offensive stat as well, and does not factor in other ways a player can be valued, in base running and fielding. Players who are valued higher because of defense can sometimes be given a misleading OPS+ stat.
Here are two quotes from a Yahoo article about OPS+:
Conceptually, it's easy to understand: it's a single number that attempts to measure a player's contributions at bat. It doesn't account for baserunning or defense and it doesn't account for the discrepancy in value between on-base percentage and slugging, but it's a pretty good indicator of how good a year a particular player has had...."
"...But it's really, really easy to use, and the percentage scale is really easy to understand. By its very nature, it's designed for the middle of the road. Many diehard baseball fans still don't really understand advanced stats like wOBA, WAR, or the much-maligned VORP, but they're often familiar with OPS+. It's easy to explain, because it's just a much-improved version of a stat we've all been using for years. Think of it as training wheels for the more advanced stuff over at Fangraphs and Baseball Prospectus."
OPS+ is a great statistic that proves a player's worth in one number. Easy as pie.
Pitching has a similar stat. It is called ERA+.