If you have a personal OneDrive, i.e., a OneDrive that belongs to you personally, then you must also have a Microsoft account, because you can't have a OneDrive without a Microsoft Account. In fact, you need a Microsoft account to post here, so you must have one.
In that case, to sign in to your OneDrive, you enter the email address and password for your Microsoft account.
However, if you get Office 365 through work - Office 365 is another feature that requires a Microsoft account - then you probably have a work-related Microsoft account, so your OneDrive is likely work-related. I don't think you want to upload your .mp4 to a OneDrive that belongs to your employer. You probably want to get your own Microsoft account.
Here is a quick explanation of a Microsoft account. (I hope this doesn't make things worse!) Your Microsoft account is a user account that exists on the internet, aka 'the cloud.' A Microsoft account has very many features, and once you sign in to your Microsoft account, you gain access to all its features, like Microsoft Store, XBox, Skype, Office 365 ... OneDrive ... and Microsoft Community, among many other features.
Here's the sneaky thing: You can, if you want, sign in to your Microsoft account directly, right here:
https://account.microsoft.com/account?lang=en-us
or you can sign in to any of its features individually. However, when you sign in to one of its features individually, that in effect signs you in to your Microsoft account, so now you are signed in everywhere.