Can you give me any suggestions on which significant accidents that were caused by a lack of poor oversight or regulatory compliance, preferably operational failures, rather than maintenance orientated?
Although the NTSB simply blamed the pilot for poor fuel management, the August 13, 2004: An Air Tahoma CV-580 freighter (N586P) crashed during approach to landing at Cincinnati/Northern Kentucky International Airport, Covington, Kentucky, killing the first officer. The cause of the crash was fuel starvation as a result of improper fuel cross-feed application.
I wrote a paper about this one which also listed poor regulation and poor certification as causes. The aircraft had a design where it was possible to cause structural damage during fuel crossfeed. All they should have done (and many did after two accidents) was fit a check valve. (Known as a non-return valve in the UK).
The FAA had allowed for a service bulletin to be issued to upgrade the fuel pumps increasing their output. The accident aircraft only had one fuel pump upgraded. When the PIC left the fuel crossfeed valve open, but changed his mind and did not switch off the high side fuel pump, that increased output pump ran all the fuel into the side with the unmodified pump, causing the fuel starvation to the engines. So, yes, poor regulation and poor certification - but don't hold your breath waiting for the NTSB to make such a finding.