OOTP has a Customizable Playoff option that can be used during the offseason or preseason in an already-created saved game. To utilize this option, navigate to: (Game Menu >> Advanced section >> Setup and Options >> League Setup tab >> Options tab >> Playoff Options section >> Use Custom Playoffs checkbox >> Customize Playoffs button. This will open a new screen where you can define your playoff structure.

There are two steps to defining custom playoffs: defining the playoff teams, and then defining how the playoffs work.

Defining Playoff Teams

The top half of the Customizable Playoffs screen lists columns of checkboxes representing each place in each of the league's divisions, arranged by sub league and division, and similar columns for wildcard teams:

Check boxes to define which teams and which wildcards you want to make the playoffs. You may choose any of the places and wildcards you wish to qualify for the playoffs. The top half of the screen is scrollable, so you may have to scroll to the bottom of the section to view all of these options in a large league.

Defining How the Playoffs Work

The bottom half of the Customizable Playoffs screen defines the structure for your playoffs:

Use the following steps to define your playoffs:

1. Using the First round matchups determined by drop-down, choose how first-round matchups should be determined. If you choose Position, first-round matchups are determined by the positions of the teams in the standings. In this case, wildcards will always receive the lowest positions. If you choose Seeding, first-round matchps are determined by reseeding the teams. All qualifying teams, regardless of how they made the playoffs, are pooled together and seeded based on their regular season records. So, with the Seeding method, a division winner might receive a lower seed than a wildcard team if they had a worse regular season record.
2. Use the text boxes to name your playoff rounds. (For example, you could have the "Divisional Round" or the "Preliminary Round".)
3. Use the drop-down to choose the number of games in each round of the playoffs (odd numbers from 1-9 are allowed).
4. Use the drop-down to define which games will be hosted by the home team in each round of the playoffs.
5. Use the drop-downs in the bracket section to define which positions or seeds should play each other in each round.
6. When you are done, click the Confirm button to finalize your settings.