How do I fit roof sheathing to an odd shape, such as a triangle?
Step 1: cut the hole into a rectangle.

If you can't do that, pick a side to be your base. This side will correlate to a factory edge of the sheet.  Ideally your sheet will be oriented correctly with respect to its strength axis. 

Measure the length if the base side. Now measure precisely the lengths of each adjacent side, from each end of your base side.

Mark the base side dimension on the edge if your sheet from a corner. From that corner, strike an arc at a distance of the first adjacent dimension. From the base length mark, strike an arc that crosses the first arc at a distance of the second adjacent dimension. If you measured precisely, this will be your third corner. There's a lot of opportunity for error with this technique, which is why a rectangle is better.

Here's a diagram that shows the idea. Ignore the labels. 



Another problem with triangles is long unsupported seams. You may want to float some backing and screw it all together to be sure that you don't put a foot through it later.