Found a good deal on a kask k50 helmet. My head measures 58cm which is the cutoff for a medium. Will a medium work for me? I've done some looking into it and it seems to work for some and not for others. No local dealers nearby so I can't try one on. Also, I don't know if I have a round or oval if that makes a difference.

I don't think this would work. I also have a 58.5 cm head, and I bought a large and returned it because it was tight.

Usually when I'm on the border, I size up on helmets.

You mean you bought a medium right? I already bought it so ill have to wait and see.

I would also err on the side of larger(winter hats,caps etc) but I hear kask has some great fit adjustments!

I'd also go with a large. I have a Kask 150, not a K50, size medium. It's about as tight as I'd want it to be for my 57 cm noggin.