Hola!
The Ohio Star block is on the same grid as a 9-patch block.
http://quilting.about.com/od/blockof...star_quilt.htm
So in a 3x3 grid with a total finished measurement of 18",
then each unit in the grid would FINISH at 6".
For the 18" (finished) Ohio Star block you'll need to cut:
Background Color (including corner units):
Cut FOUR 6.5" squares for the corner units
Cut TWO 7-3/4" squares for the stars (for QST)
Star Color (including center block):
Cut ONE 6.5" square for the center block
Cut TWO 7-3/4" squares for the stars (for QST)
(QST - Quarter Square Triangle )
The instructions provided in the above link show you how to make the QST blocks from the larger cut squares, and assemble the components into the Ohio Star block.
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Just a side note on making QST blocks - and not to confuse the issue (some people do it this way and others have specialty rulers and others cut perfectly every single time - I'm not in the last group):
I prefer to cut my QST blocks oversized and then trim down.
In order to do this, the starting squares should be cut larger than 8".
The exact size doesn't matter, as long as you have a square of each color and they're the same size.
Make the QST blocks in the same manner as the link above, but then trim them down to the exact UNFINISHED size (6.5") you need.
Hope that helps.