I think
Rosamund "
Rosie" works even if
Rosamund itself is pronounced with a short o. Kind of like Cat/Kat work as nicknames for Catherine/Katherine even though the nicknames have a t sound while the full names have a th sound.
masculine list:
https://www.behindthename.com/pnl/191050/124079feminine list:
https://www.behindthename.com/pnl/191050/124080