You possibly cant get a better located apartment in Madrid. Oscars apartment is bang in the centre of the city; You hop, skip and jump and you'll be in Plaza Mayor, which is the heart of Madrid. The apartment itself is very well designed and themed in black and white (which we loved). Also, Oscar has been very helpful as a host and was very responsive whenever we called him for anything. The place can easily accomodate 4 people.
The only downside we found of this place was that it is on the 5th floor and there is no elevator in the building. For the 4 of us, it wasnt a problem since we had very little luggage. But if you cannot climb stairs or have heavy luggage then it may be a problem for you.
All in all, we loved our stay at oscar's place.