The following is a list of the locations used by the Master MOS. The information is (I believe) correct for Version 3.20 of the MOS, ie the most common version. I suspect that it will not be very different for other versions of the Master MOS. Some locations certainly are different in the MOS versions used by the Model B and B+.
A8 - AF General "*" command workspace.
B0 - CF Filing system workspace. The locations between C0 and CF must be preserved between filing system calls.
267 Startup message and !BOOT error action status. Note: There is an error in the description of OSBYTE D7 in the Reference Manual.
26D-270 Interpretation of input values C0 to FF (values not normally generated by keypresses). Each byte controls a block of &10 values. Meanings: 0 - ignore; 1 - use soft key expansion; 2 and above - return values from this number upwards.
276 Escape side-effects status (0 - enabled, non-0 - disabled). See documentation of OSBYTE &E6 for what the effects are.