In economics, the attributive adjective year-over-year means compared to the same time last year. Examples: "Year-over-year sales show a marked improvement." "Expenses continue to fall year over year." (The hyphens disappear when the adjective is used predicatively.)
I have only one citation, but it appears that the term has broadened its meaning inside Microsoft and is now merely a synonym for annual or year after year.
We hold decision-makers accountable year over year for carrying out their plan.
There is no obvious compared to the same time last year going on here. It's not like you are 15% more accountable this year than you were last year. Rather, the sentence merely says that reviewing how well the decision-makers are carrying out the plan takes place every year. (It may not be clear from the sentence above that that's what the sentence means, but it's clearer in the context of the entire document from which the sentence was extracted.)