OpenSystem (株式会社オープンシステム) was an affiliate company of Human based in Tokyo. It allegedly had a development room in Osaka, in the same building as Human's Osaka branch, and much of the staff came from Osaka-based G.O 1, the company behind The Golf '92 for the Famicom. 
Forum appears to have been a brand used by OpenSystem, but Human later used it for two PlayStation games, Universal-ki Kanzen Kaiseki: Pachi-Slot Simulator and Warau Fukei-san Pachi-Slot Hunter. (Incidentally, both games were developed by Media Entertainment.)
Research Methods: Actual mentions
- Formation Soccer on J.League (JP Publisher: Human)
Super Famicom/Super NES
- Finalset (JP Publisher: Forum)
- Super Indy Champ (JP Publisher: Forum)
1. Szczepaniak, John. The Untold History of Japanese Game Developers: Volume 2: Monochrome. United States: SMG Szczepaniak, 2015. Print.