DO's officially finished on August 31st 2014. Also the travelling overseer upper limit of age 70 came in then and so that naturally culled a lot of them along with many CO's too.
The DO's used to usually get drawn from the ranks of the CO's. They oversaw the District conventions, went with the CO twice a year to congregations to "encourage" and train them.
They would also ride shotgun with a regular CO on more difficult or multi-circuit "problems".
Now the longer serving and more capable CO's oversee the REGIONAL conventions and also go with the less experienced CO's to "problem" congregations but less frequently than twice a year.