The greatest one-season improvement in MLB history is held by the Diamondbacks. They went from 65 to 100 wins in their first 2 years of play. The O's have currently won 11 more games than all of last year with 41 games left. They need to go 25-16 to claim the #1 spot for most improved team ever. Given the remaining schedule, that seems like a dream.
But they've been a dream already.