OPG is not a centrally selected catalogue of problems. Any registered user can publish and map a problem using OPG's graph-based tools. The goal is to coordinate work around problems contributors actually bring to the platform, not enumerate every open problem. That being said, every Erdős Problem, every Openproblemgarden.org problem, famous open problems like MLC (Mandelbrot set locally connected) and the Millennium prize problems...that's a good start. I'm personally adding these problems in due time--right now, there's only a subset.