I am replacing all the wood trim around my garage door openings. The trim butts to vinyl siding with j-channel. The siding at the top of the doors had no flashing or trim cap and that is where the water got in. I am installing flashing above the door trim. The wall itself only has chip board (I guess it is really called OSB) behind the siding. Should I install some sort of tarpaper or plastic behind the siding a few inches along the top and sides of the opening before putting up the new wood trim? Seems like another disaster waiting to happen with just bare chip board behind the siding. I pulled out a bunch of soggy chip board where the leak occurred at the top. The chip board had soaked up the water like a sponge! North facing wall which gets snow and ice in winter. Moss growing in J-channel above door due to lingering moisture.
I would put some flashing there. I was just doing some work on my back door and while it leads directly to the house not the garage. There was some flashing that went under the siding and over the trim. (For the horizontal piece) You're right OSB does soak up water pretty well. Once wet it doesn't stay together very well either.
This is one of the drawbacks to what I refer to as "the caulkless era" of exterior construction, before housewrap and other drain-plane measures were written into code. All those channel joints with no protection from driven rain (and your header trim sans flashing to boot).
Consider the drain route for water coming off the flashing you're installing. Unless it has a positive slope forward, much of it's likely to run to the ends and down the side trim. Do what you can to properly lap a water barrier behind the flashing and down beyond the sensitive materials on the wall.
For more help, please post a photo.