The metal that occurs in vast limestone deposits and combines with oxygen to form an oxide with the formula MO is calcium (Ca).

Here's why:

* Limestone: Limestone is primarily composed of calcium carbonate (CaCO3).

* Calcium Oxide (CaO): When limestone is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO2). This process is called calcination.

* MO formula: The formula CaO fits the general formula MO, where M represents the metal.

Therefore, calcium is the metal that fits the description.