I have a rather sizeable hole at the top of my foundation (just below the sill plate) where the vent and fill pipes for an oil tank used to pass through. It's about 6 inches wide and 4 inches tall. The guy who removed them really chipped away at the wall to get them out.

Anyway, now I need to figure out how to fill this hole. The only way I can think of doing it is to create a form. I figure I'll block the inside end of the hole with plywood, and do the same on the outside, leaving just enough room at the top to pour in the concrete. This would, however, still leave a gap at the top.

Anyone ever done this before? What type of concrete should I use?

The hole should be totally covered from one side like you described. The other side I'd cover with 3-4 thin strips of lath of dimensional lumber adding a new board as you fill the cavity. The last board can be tilted out so you can add the last bit of hydrolic cement and push in to compress.

Cover one side of hole. Cut a piece of plywood as in A. Nail on 2x4 as in B. Cover with adidional plywood as in C.
Tack assembly to wall or prop in place with stakes. Top of assembly should be above top of hole. Mix concrete "loose" or wet so that it flows freely. After filling hole with concrete, adigitate concrete with small stick or rod to make sure that it fills all the crevices.

Let dry for an hour or two. When concrete has firmed up enough to hold its shape remove form and cut off excess concrete with trowel. Moisten surface slightly and smooth with trowel.

Thanks, Harold! Amazing!