Tell me what is wrong with this of code

buyandsell.go

type Cement struct {
	NoOfCement int
}

func (c *Cement) BuyCement(howmany int) {
	c.NoOfCement += howmany
}

func (c *Cement) SellCement(howmany int) {
	o.NoOfCement -= howmany
}

func (c *Cement) String() string {
	return fmt.Sprintf("%v", c.NoOfCement)
}

func main() {
	var cement Cement
	cement.BuyCement(15)
	cement.SellCement(9)
	fmt.Println(cement)
}