• Recall the mnemonic GEROA which means Gain Electrons is Reduction = Oxidizing Agent. This tells us that whichever gains electron/s will be considered as the OA
• Analyze the reactants and product's oxidation number to find who gained electron (became more negative)
• For the reactants:

Na(s) + H₂O(l)

• Na in Na (s) is 0,  recall that oxidation number of an element in its natural state is zero
• H₂O: H is +1 (H with nonmetal is considered +1) and O is -2 (O is usually -2 except in peroxides and superoxides)

• For the products:

NaOH(aq) + H₂(g)