Is there a way to measure your fork to see how wide of a tire will fit? I have a ocr3 and would like to move up to at least 700/28. I make all of my parts purchases online, and would hate to get a too wide tire.

Measure the tire you have now. If it is a 23 that should be the width in mm, but this is not always exactly true. See how many popsical sticks you can fit between the tire and fork/frame both at the top and the sides of the wheels, for both the front and back wheels. The difference between a 23 and 28 is 5 mm at the top and 2.5 mm at the sides.

Also, some 28s are bigger than others. Panaracer pasela 28's usually fit the spacing on a "racing" bike, while I've had specialized and hutchinsons rub even though they are all supposedly the same size.