Mainfig is a file /plugins/OMGPI/main.omgc. It manages main OMGPI options, for loading games or database.
selectedGame: random
lastGame: OMGPI_SW
  hostname: localhost
  port: 3306
  database: db
  username: admin
  password: 1234
area: '-500,0,-500,500,255,500'

Selected game is a GameName.jar file, or random to load a random file. You may want to use /setgame <JarName> instead to set it.

Last game is used by OMGPI so in game rotation games don't repeat. But you can still set that to a jar name, so it loads any except this one when having more options than just it.

MySQL is setup with things you got from your mysql supplier. Notice: you cannot use more than one database at a time.

Area is the game area. Players moving out or outside will recieve a "You can't go so far!" message and get moved back. The value is a cuboid made out of 2 points, like x1,y1,z1,x2,y2,z2. You might be familiar with it if you ever worked with WorldEdit.