The formula KNO3 represents potassium nitrate, a compound composed of potassium (K), nitrogen (N), and oxygen (O) atoms. To determine the oxidation number of oxygen (O) in KNO3, we need to consider the charges of the other atoms and the overall charge of the compound.

- Potassium (K) is a group 1 element and typically has a +1 oxidation number.

- Nitrogen (N) is a group 15 element and can have various oxidation numbers. In KNO3, nitrogen is bonded to three oxygen atoms, so it is likely to have an oxidation number of +5.

Let x represent the oxidation number of oxygen in KNO3. Since the compound is neutral, the sum of the oxidation numbers of all atoms must be equal to zero.

1 (+1) + 1 (+5) + 3x = 0

Simplifying the equation:

+1 + 5 + 3x = 0

6 + 3x = 0

3x = -6

x = -2

Therefore, the oxidation number of oxygen (O) in KNO3 is -2.