First, balance the equation: 2 CO + O2 --> 2 CO2.
Then, compare the coefficients of CO and O2.
You can see that it takes twice as many moles of CO as O2 in order for this reaction to occur (2 mol CO/1 mol O2 ratio).
So, there must be twice as many liters of O2 as well.
11.8 L O2 * (2 L CO/1 L O2) = 23.6 L CO needed