You take control of your favorite team as the GM or manager (or both) and try and steer them to World Series glory. It's a slow burner—I played a fair amount of OOTP 19 and it took me a full five seasons to turn the Toronto Blue Jays around. If you want, you can watch every at-bat of every ball game, but you'll most likely be simming through the season to see what impact your trades, call-ups and strategy changes have had.
OOTP 20 adds a few features we haven't seen before. The first is a "live" approach to the baseball season: you'll be able to jump into a career at any point during the upcoming regular season and begin with up-to-date rosters, stats and standings. The dev team has also revamped the animations for the 3D sim engine, added new strategy options as well as the national leagues from Germany, France, Spain and the Czech Republic.
Perfect Team Mode, in which you battle against other managers online, returns, and it has microtransactions linked to its card packs—although I couldn't say yet how annoying they are. A couple of Steam players have mentioned them in user reviews, but the majority of the ratings are positive.
If you're looking for a baseball sim that lets you take to the field, check out Super Mega Baseball 2, which is still very good.