So is the original requirement for O_DIRECT addressed completely by O_SYNC and O_DSYNC?

The way I was told it, if the database engine implements it's own cache (like InnoDB and presumably Oracle), are just "doubling up" if you also use the OS cache?. Perhaps Oracle is happy with its own internal caching (for reads).

I've seen a DB guy insist on O_DIRECT without implementing array controller cache battery alerting, or checking if drives themselves had caches disabled. Nope "O_DIRECT fixes everything!" ... although these days enterprise class SSDs have little batteries and capacitors to handle power loss so in the right circumstances that's kinda resolved too, but like the array controller cache batteries, this is one more thing you have to monitor if you're running your own hardware