We demonstrate 4 × 10 Gbit/s error-free bidirectional transmission over 2 km of conventional OM1 graded-index multimode fiber using OOK modulation and direct detection. We also perform field transmission to show reach and capacity boosts on legacy multimode infrastructure. Such transmission is enabled by selective mode group division multiplexing, based on multi-plane light conversion over 4 mode groups of the multimode fiber.