on = on a surface.

For example,

• There’s a clock on the wall.
• What’s that on the ceiling?
• There’s a bag on the floor.
• My books are on the table.

on = not exact address.

For example, I live on Oxford Street. (However, for an exact address, you say “I live at 55 Oxford Street.")

on = attached to.

For example, She is wearing a ring on her finger.

on = close to a river.

For example,

• London is on the River Thames.
• My house is on the River Avon.