I found how BBR does their OPS+ calculation, and I'm not sure I agree with it. Instead of calculating the OPS for both the player and the league and then comparing the two, their calculation calculates OBP+ and SLG+ separately, adds the ratios together, subtracts 1, then multiples by 100.
'55 Aaron: .366 OBP, .540 SLG
NL Avgs: .328 OBP, .407 SLG
Using this example, I don't get 141 but it's closer to 141 than 123.
OBP+ = 111.5
SLG+ = 132.7
OPS+= 144.2