First allow me to say congrats on having access to these nice items. Now for the bad news, depending on the type of plastic that the drum is made of you might not ever get the olive smell out of them. If you want to give them a shot I would say hot soapy water, filled about 1/4 of the way, put the lid on and roll them around to agitate and then rinse.
Few blogs i have read state that once you store food in a plastic barrel, you will never get the smell, taste, or bacteria out. Even with soap, water , and bleach it will not get everything.
Try cleaning it your best, then fill it with water. Add enough chlorine to neutralize it and let stand for 2 weeks. After the 2 weeks open her up and give a whiff and let us know if you would still drink that water.
5 gallon buckets from any walmart bakery, company wide any location they are $1, they stored donut frosting in them. So far I have bought 10 of the 5 gallon ones. A little soap and hot water and all smells were good to go. Food safe, I put beans and rice, and even some bug in cat and dog food in them. I will eventually start to use these for water storage as well since 5 gallon buckets are easy moved in comparison of larger sizes.
If I could source some food safe 55 gallon drums preferably in the plastic variety I would use those for water instead, but as of this moment no where in NE Alabama has anything of the sort in a public manner at least.
I got some that had hot peppers in them back when everyony was worried about Y2K. I scrubbed them out with hot soapy water and rinsed them out. Then I filled them with water and added a qt of bleach. Let it stand for a week and dumped it. No smell.