o bechgyn / o fechgyn
If the following prepositions are used as they are, without an ending, they will cause a soft mutation. The following prepositions cause a soft mutation when followed by a noun:

• am
• ar
• at
• heb
• i
• o
• dan
• dros
• drwy
• gan
• wrth
• hyd

e.g.
gwrando ar raglen radio
talu am ddau docyn
deg o ferched.