@Stuart ********
This is so confusing. I am in the situation that the first year of my multiple entry O-A visa is almost over. I had planned to reenter and get a stamp for an other year.
However, the consulate in Frankfurt told me explicitly that this is not possible and I would only get a stamp for the day of my first entry on this visa + 1 year. So I don't get a 1 year stamp at reentry, I can only stay for 1 year counted from the day of first entry. I also talked to an experienced visa service and they said the same.