Spun up a quick image showing just where akami, chutoro and otoro are cut from a Bluefin Tuna.  ”Otoro” allegedly can only be taken from the fattiest bellies of Kuromaguro and Minamimaguro Tunas.  So, please assume the diagram below is of a Kuromaguro which  has a thick layer of fatty flesh between its akami and chutoro innards and the ocean (separated by the white dotted line).  You may hear of “Toro” in general (with the “O” dropped), which refers to fatty maguro meat with the gelatin-like sinews that are rich in collagen and is present in most sushi shops, grocery stores and fish markets.  Sushi shops normally don’t stock a lot of Toro or Otoro since it doesn’t pay off as well as cheaper-to-procure high-selling fish like kohada, buri, sake, etc.

(Edit: Otoro is cut from the underside of the fish (the belly) while chutoro is cut from fatty parts closer to the dorsal region.  Please refer to the image below more like a color guide, not as an anatomically correct cross-section of a Bluefin maguro.)