What is the correct way to spell my name for the train ticket?

My name has a letter ö (o and two dots above it), and it is written that way in my passport. I guess I can't use that letter for the booking. Do I just write it as a normal "o", or change the letter to "oe" as is done in the machine readable part of my passport? Could this become a problem if I write it the wrong way? Thank you for help!