As long as you've kept good clean oil in it, and it doesn't get oil dirty right away, I wouldn't mess with a "flush." I've heard those flush chemicals can cause more harm than good to gaskets and seals... just keep good oil in it at regular change intervals. 200k+ miles isn't a problem if it's maintained.
And O2 sensors can indeed go out at same time. If they're original, change them.
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