So a weird RAM corruption issue I was having turned out top be because, in my decoding logic, I was qualifying the r/w and oe lines with PH2O. When I switched to using the actual clock line for this, the system runs perfectly. Strangely, when looking at my rw and oe lines, compared to the true clock, things looked perfectly correct (oe and rw only happening on HIGH clocks, and edges looked pretty properly aligned to my eye). I guess I created a timing issue of some sort.

This incident leads me to a greater question. What is the purpose of the PH2O line? When would I want to use it? according to my scope, PH2 and PH2O are only out of phase by 5 nano seconds or so.

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To expand on that, according to the 1975 datasheet the Phi1Out and Phi2Out are non-overlapping, and reflect the clocks used within the chip. Mostly, 6502 output timings are relative to Phi1Out rising, and input timings (and the data bus as output) relative to Phi2 falling.
http://archive.6502.org/datasheets/mos_ ... g_1975.pdf

As BDD implies, whether it matters as to which clock you use will depend on whether the design is using an original NMOS 6502 or a modern CMOS 6502.

I suspect it will also depend on the behaviour of the RAM: in the 70s and 80s RAMs were rather slow. Modern RAMs are much faster. It might be that a bit of slosh in the circuit timing will be tolerated by the slower RAM, but the faster RAM will react, to bad effect.

You'd need a good scope and test program to investigate what the RAM is actually seeing.

So: which kind of 6502, and what speed of RAM, are two major questions when you have this kind of timing issue.

Would PHI1O and PHI2O be part of the 6501 / 6800 socket compatibility legacy ?

Yes, I think it's related. If you look at the block diagram in the '75 datasheet I linked, you'll see that the single clock in is fed to a clock generator and the two derived clocks are used and also exported. It's the bringing on-chip of the clock generator which was a major step forward for the 6502, and the earlier systems would have had those two clocks available as references.