I had OSG (and chondromalacia patellae) in both knees
The absolute best thing you can do is mobilise everything up and downstream of the knee. That means soft tissue work on the tissues around the knee (all quad insertions, hams, calfs, anterior/posterior tibialis). That's a big job on it's own. Then look and your ankle, making sure you have full range. Then your foot; do you have a collapsed arch/dropped navicular bone? Your foot might require some specialist attention, my physio is also an osteopath and he's done some good stuff for my old sprain.
Next up is hips, which is potentially a huge task is you're particularly broken. Hips need to be mobile and stable, so you'll need to work through the relevant stretches/mobilisations in this thread (glutes, hams, quads, anterior hip) AND work on more dynamic unilateral leg exercises and on strengthening your glutes. Strict DB walking lunges (upright torso, almost vertical front shin), overhead walking lunges, cable pull throughs, glute bridges, controlled step ups... Hips need to be level at all times for all of these, and you can't have any movement in your lower back.
That was a bit of an explosion of info, so if you have any questions just ask.