Silicone sealant is usually used to seal the joint to avoid water and wind infiltration. The tough decision is the color. I think I would try to match the brick in order to avoid a noticeably irregular outside edge.

a backer rod and caulk is typical in that situation. We will usually either match the mortar or the brick, your call. In this situation I would probably match the mortar since the color is close to the frame color and it would look like a continuation of the frame. Your call though.

If caulked nice, it will look just fine and you will never notice it.

ditto. gaps over 1/4" need backer rod to fill
in, caulk over backer rod.

you need something to expand & contract
that will adjust to movement of
window, wood & brick.

a filler piece of wood is just covering
up the gap, but if you wan't to do that..
make sure caulking is done before hole
cover in installed.

best of luck.

the window frame is white, but you are suggesting white might look bad, Renovator? Do they make caulking the color of this brick? thanks!

we tpyically spec Sonneborn NP-1 caulk for exterior applications. they have a bunch of color options that can be had. You should be able to get one very close to the brick color.