A few numbers
The approximate volume of the Gulf of Mexico is 6.43 * 10^17 gallons. Not the whole ocean, the Gulf of Mexico.
The estimated top rate of the oil leak is about 800,000 gallons a day.
An Olympic swimming pool is about 660,000 gallons.
If the oil leak lasted 100 years at the current rate, it would be the equivalent of pouring a little less than four ounces - half a cup - of oil into that swimming pool.