Its % based

Sir, I believe you are numerically challenged. :)

Example

League Average OPS - 750
Player A OPS - 2000

This is an OPS+ greater than 200.

In fact a perfect OPS is 5000, so an OPS+ of much higher than 200 is possible. (if extremely unlikely)

An ERA of 0.00 in the context of a typical league average ERA would yield an ERA+ that was much greater than 200.

FYI... those stats also take park factors into account. So, guys who hit in Coors Field (or at least in the pre-humidor days) will take a hit in OPS+ while players in Shea will get a boost.

ERA+ is 100*lgERA/ERA where lgERA is park-normalized.  

An ERA of 0.00 results means that ERA+ is undefined ("infinite" if you will as an ERA of 0.01 would result in an ERA+ of a few thousand) and a 0 ERA+ is only possible if the pitcher has given up earned runs without retiring a batter.

OPS+ is not scaled OPS, but its OBP+ plus SLG+ minus 100.  So, in full, its:

100*(OBP/lgOBP + SLG/lgSLG - 1) where lgOBP and lgSLG are park normalized.  Because of this interesting definition, a batter with OBP and SLG of .000 has a negative OPS+! (-100)

Interesting factoids:

-- Because ERA+ has ERA in the denominator, you cannot do simple weighted-by-IP averages to find career ERA+ from seasonal ERA+ values.  A harmonic mean is necessary.  For example, the average ERA+ of two 200 IP seasons of ERA+ 100 and 200 is not 150, but is 133.

-- OPS+ is indeed an odd formula, but it works.  It more closely approximates the runs created formula of OBP*SLG because it removes the difference in scale between OBP (~.330) and SLG (~.430).