Strictly speaking, that round metal mounting bracket on the back of the fixture is supposed to be secured to an electrical box.
It can be a shallow box, but it's supposed to be a box. You must protect the wires. You want that new garage to still be there 10 years from now.
I can't really tell from the photo, but the right shallow metal box should fit within that recess on the back of the fixture without you needing to cut a hole in the siding. But if you have to cut a hole, then you have to cut a hole (if you do, rent, borrow or buy a holesaw for your drill big enough for the box).
You could attach the box through to the OSB with screws, although personally, I'd lay a 2x4 flat side against the OSB inside the garage, toenail it into the studs on both sides, and screw the box directly through the OSB into that 2x4. So a stiff breeze or errant basketball doesn't knock your light fixture off the wall. :-)
Drill a hole through the OSB and the 2x4 big enough to feed the cable and to accommodate the clamp that holds the cable in the box. The cable has to be clamped into the box.
Then attach the round mounting bracket that came with the fixture to that box, wire everything up, attach the light fixture to the bracket that you attached to the box. That's the way it's designed.
If it's a metal box, it has to be grounded (screwing the grounded mounting bracket to it will accomplish this).
You also have to secure the cable inside the garage as dictated by your local electrical code.
Finally, I'd use a dose of waterproof silicone or caulk around the box, and then again around the fixture itself after it's mounted, to make sure water doesn't seep inside.
Good luck!
I know exactly that model and it's a cheapie! Yes, that middle panel pops off easily, just get into the slot where the middle panel touches the edges and start prying. It will pop right out.
This type doesn't have a ballast proper, just some discrete components placed in the end caps and the gutter you're about to expose. Very hokey. I do not believe it would play well with a plug-n-play LED tube.
You can of course gut that stuff and rewire it for direct wire LED tubes. I recommend the kind of direct-wire that takes hot and neutral on opposite ends. Those endcaps with the integral tombstones are pretty cheap, and I would not want to have 120V between the adjacent pins.
Best Answer
You need to adjust the screws so they protrude from the front of the bracket just enough to secure the fixture to the wall with the included cap nuts. Then cut off the part with the screw heads, flush with the back of the bracket. Good luck.
Many wire cutters/strippers have the ability to cut this size bolt and protect the threads.