OP is looking at a sea of disappointments if he persists in a belief that there's anything 'seamless' about OneWorld, lol. There are seams everywhere, many of them unraveled.

One thing that is somewhat effective, though, is the ability to book award travel on one OW carrier using miles from a different carrier's account. Not necessarily with 'native' availability or pricing, but generally doable.

But finding and booking non-native flights can be a bit challenging. Native to AA, I sometimes use BA avios and see non-BA availability (including on AA within the U.S.) so if OP looked for BOS-MCO award seats and didn't see any there probably weren't any. Trying that request on aa.com will answer that question.

BTW AA appears to value its FF miles at about a penny, for coach domestic travel. Thus if a 'saaver' award is available for 12,500 miles, you'll find a cash fare for about $125.