Since O Scale stadiums are rather large, the stadium seating pieces that we designed are in S Scale. Each has a 10-seat configuration and measures 3-1/4" long. That equals 17'-6" in S Scale and 13'-0" in O Scale.
 
This single kit includes enough parts to make 12 sets of  a 10-seat row  configuration (or 120 total seats). Each row consisting of two parts, one is the seat surface and the other is the seat back surface. One slots into the other and the row is easily assembled with Tacky Glue. Material is PolyBak (Laserboard).
 
Assembled seat rows are easily mounted, primed and painted.
 
For larger quantities, consider the Bulk Seat Kit which produces 1,440 seats.