One of the many new features in Android O is support for aptX Bluetooth streaming. aptX is a series of streaming protocols owned by Qualcomm, that feature much lower latency compared to normal Bluetooth audio streaming. But it looks like Nexus 5X and 6P owners won't be able to use it.
On the Google Issue Tracker, one user created a report explaining that the aptX option was missing on his Nexus 6P. After some back and forth, a Google developer labeled the bug as 'Won't Fix', with the below explanation:
Status: Won't Fix (Intended Behavior)
Unfortunately support for aptX and aptX HD are not supported on Nexus 6p and Nexus 5X - only on newer devices.
Of course, the big question is this: is Google just unwilling to add support for aptX, or is it technically impossible? In a similar case, Google initially didn't bring the Pixel's fingerprint sensor gestures to the 5X and 6P, due to the older devices having older fingerprint sensor firmware. After a bit of public outcry, Google began evaluating if updating the sensor firmware was feasible, and it eventually came to the 5X/6P as part of Android 7.1.2.
It's difficult to say for certain if aptX support is possible with the Nexus 5X and 6P. According to iFixit, both phones have the Qualcomm WCD9330 audio codec chip. That component is also included in the LG G4, which does support aptX (but also in the OnePlus 2, which doesn't). This isn't conclusive by any means - a license is also required, and possibly other software/hardware components. Qualcomm doesn't make it obvious on the aptX site what it takes for a device to be compatible.
It's always annoying to see older devices left out of newer software features, but that's technology for you.