Capitalizing prepositions in abbreviations?

Solution 1:

There is no "rule" that the letters making up initialisms must be all one case. In Britain, Transport for London is abbreviated to "TfL", and the Court of Protection can be either the "COP" or "CoP". Initialisms which are also acronyms, like scuba, laser, radar, etc, have become words in their own right. As you have found, individual organisations have their own conventions about capitalising, and you would be wise to adhere to them.

Solution 2:

You have answered your own question: "Should I stick with my weird OoO?"

If a mixed-case initialism is weird, then why stick with it?

When handwriting such an abbreviation, it makes little difference. With the ubiquity of electronic communication, there is a difference: WFH takes four keystrokes ShiftWFH but using a lowercase letter in the middle takes five: ShiftW F ShiftH. Why make work?

As Michael Harvey observes, some organisations will have their own style. Transport for London not only uses a lowercase f but italicises it: TfL. And it's even like that on manhole covers.

Just do what's usual. OOO and WFH are perfectly normal and expected. If you're working somewhere where they're not, use what is.