How do you calculate obs in baseball?

This is simply on-base percent plus Slugging. It is a stat similar to Gross Production Average -- it gives a number. The higher the better,

on-base percent is: (hits + walks + hit by pitch) / plate appearances.

Reaching on an error or a fielders choice does not count as an on-base. A sac fly does count as a plate appearance.

Slugging is: (1*Singles + 2*doubles + 3*triples + 4*homers) / total At Bats.

sacs and walks do not count as at bats.

The theoretical max for on-base percent is 1 (assuming you always get on base)

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.