Silver ore is oxidized using oxygen from the air, as shown in the reaction below :

$$ 4\text{Ag} + 8\text{NaCN} + 2\text{H}_2\text{O} + \text{O}_2(\text{air}) \longrightarrow 4\text{NaAg(CN)}_2 + 4\text{NaOH} $$

Here, silver ($ \text{Ag}(0) $) is undergoing oxidation to form $ \text{Ag}(+1) $ :

$$ \text{Ag}(0) \xrightarrow{\text{oxidation}} \text{Ag}(+1) $$

Silver is then precipitated from the solution by adding zinc dust in a finely divided form :

$$ 2\text{NaAg(CN)}_2 + \text{Zn} \longrightarrow \text{Na}_2\text{Zn(CN)}_4 + 2\text{Ag} $$

In this reaction, $ \text{Ag}(+1) $ is reduced back to $ \text{Ag}(0) $ :

$$ \text{Ag}(+1) \xrightarrow{\text{reduction}} \text{Ag}(0) $$

In summary, oxygen from the air acts as the oxidizing agent, and zinc dust serves as the reducing agent.