How should I install underfloor heating over OSB subfloor?
Skip the OSB - attach the tubing to the insulation. XPS insulation plays well in concrete/cement applications. OSB Does NOT. I can't imagine what lead you to this particular "non-traditional" idea, but run away from it before you find your whole floor disintegrating under your feet.

Responding to comment - the insulation glued to the slab should not have any appreciable gaps - if it does, fill them with more insulation. If it is, in fact, XPS (extruded - as it should be - closed cell foam, smooth-surfaced, usually pink, blue or sometimes green) rather than EPS (expanded - crumbly white stuff) it stands up to foot traffic just fine. Tubing clips are typically screwed right into it. If you have glued EPS to the slab, then you'd be well served to buy the thinnest sheets of XPS you can find (1" or 25mm is typically the thinnest) and place that over the EPS, without leaving any gaps this time. It usually has a tongue and groove edge treatment to help with that. 

These are folks to whom my relationship is "just a happy customer" with plenty of detailed information about installing radiant heat in a manner that will last. I installed mine over 2" XPS, walked all over it running tubing and reinforcing steel, and didn't make so much as a dent in the insulation. That page has links to both a written manual and a video.