Electrical – For installing a new exterior light on a brick wall, can I connect the wires in a junction box mounted on the opposite side of the wall

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!

EDITED: Now that you've posted a picture... ;-) Still test to confirm this, but it looks to me as if:

• The 2 blacks and 1 white that are connected, are feeding power through the light box to downstream outlets and also sending power to the switch (on the white wire). Wrap the end of this white wire with black tape or mark it with a black permanent marker. It's a hot wire.

• The lone black wire with a wirenut on it is the power coming back from the switch. This one is hot when the switch is on, dead when the switch is off. Connect this to the black/hot lead on your light fixture.

• The 2 white wires are the neutral (grounded conductor) from the incoming circuit, and the neutral serving the downstream outlets. Connect these to the white/neutral lead on your light fixture. I'd probabaly add a short pigtail so there's only one connection to the neutral on the light fixture, and also so it's that much more obvious which wire(s) connect to the fixture.