Hi. JALhad posted little info on OVS BASEs on their FB long time ago.
The routes/flights each base flies changes every 3yrs or so. Or, with new route opens, shuffle happens.
Not sure if info outdated, here is interesting info on SIN base.

Historically, YVR by HKG / LAX by SIN. But when JALways merged, BKK takes LAX. (NGO/KIX-BKK/HNL:Thai chief purser available since then. )

with recent ant expansion of networks, shuffle continues (CDG by Asian crew, SYD by LHR etc)