9 3/4" clearance between bottom and second shelf. 10" clearance between second and top shelf. We are only using the middle shelf for three items that weigh less than 20 lbs total, but we have a big old Hitachi receiver that weighs a lot, and I don't think putting it on that shelf would be a problem. However, consider the depth as well. Our Hitachi is 14" deep (including knobs). The center and bottom shelves are just over 15" and 16" deep (before they hit the hardware) respectively. If your Onkyo is deeper than that, then you might want to reconsider.