My 3rd grade son was involved for the first time last year. His school does not compete in the K-2, but there were teams at our regional competiton. At his school, due to the popularity, they have "try outs". Last year, they chose four teams of 7 students each and began practicing October/November. Each team competed on a different problem. The regional competition which is one day was in March. They won first place and advanced to the state competition which involved travel for the weekend which they also won. I'm not sure if the K-2 advanced on from state. I don't remember seeing that age of kids at the World Finals. We were at the University of Maryland for 5 days for the finals. It was really an unbelievable experience for the entire family. I think it challenges those students who are creative and enjoy problem solving.
My best advice is to maybe check out your regional OM website to see who placed in the K-2 category. I would contact the school and see if there would be someone willing to discuss with you the program. I know that when my son's team was practicing for the World Finals, an elementary school that was considering starting the OM program came to watch the practices. Also, if you could obtain videos of other school competitions to review. As a parent, I didn't quite understand OM until I sat and watched different teams competing.
Best of luck! When my son was in kindergarten, his teacher asked me to consider starting a K-2 team but I teach high school and just didn't feel I could commit the time needed. There really is lots of practice and building time involved. Make sure you have great parents to help!