Option 1: Magnesium - Magnesium reacts with oxygen to form magnesium oxide (MgO), so it does form an oxide on reaction with oxygen.

Option 2: Lead - Lead reacts with oxygen to form lead oxide (PbO), so it does form an oxide on reaction with oxygen.

Option 3: Tin - Tin reacts with oxygen to form tin oxide (SnO2), so it does form an oxide on reaction with oxygen.

Option 4: Silver - The correct answer. Silver does not react with oxygen to form an oxide. Unlike most metals, silver is quite unreactive and does not readily form compounds with oxygen.

In conclusion, out of the given options, silver is the only metal that does not form an oxide on reaction with oxygen.