More wires than Nest needs
The blue wire goes to C, not B

Some Trane systems, yours included, use a non-standard labeling where the 24V common wire is labeled B, not C.  We can tell this from your wiring diagram as well as the wiring on your old thermostat.  As a result, since the blue wire went to your old thermostat's B terminal, it needs to go to C instead on the Nest.  This frees up the O/B terminal on the Nest to get connected to the orange wire from the thermostat, which connected to the O terminal on your old control.

As to the brown wire? While it might be possible to figure that out with more information, your system should work fine without it connected, since it was not hooked up to your old control.  The white wire being unhooked, though, is a sign that the emergency/auxiliary electric heat was not used on the old system.  Perhaps it was disabled intentionally due to an issue with it, so I would pay attention for signs that the emergency heat is running when it shouldn't.