Read How OneDrive safeguards your data, their privacy statements, and third party reviews of their service. And come up with your own conclusion.
Microsoft employees doing “whatever they want” is not how it is supposed to work, as MSFT states:
Access control systems: OneDrive and Office 365 maintain a
“zero-standing access” policy, which means that engineers do not have
access to the service unless it is explicitly granted in response to a
specific incident that requires elevation of access. Whenever access
is granted it is done under the principle of least privilege:
permission granted for a specific request only allows for a minimal
set of actions required to service that request.
However, its remains possible that a bad actor does something untrustworthy, or their systems are compromised through technical means. Up to you to evaluate the risk of how likely you would be targeted, over their many other customers.
Oh, and it doesn’t take a Microsoft engineer to read your files. Third party apps, if approved by you, can do that. Malicious Azure apps exist to exploit this. Manageable by being careful what you allow. But think about what has more controls over it: the privileged access procedure promised customers, or a public app store where any random person can write code.