Overall O&V's apt meets all the minimum requirements from someone who wants to explore Vienna and who doesn't need a luxury room or services.
It's located close to the main touristic sights, so we usually got there and back on foot; the apartment is big and very beautiful - we spent quite some time staring at different objects and pictures in their living room. In 2 min range, there are restaurants that open early and close late - you won't die from starving. You can't use O&V's kitchen, btw. The apt is very quiet all the time.
A guest room is also nice and filled with life, with shelves full of books about arts and tourism; the ceiling is sloped and quite low (~210cm) even in places where it's not sloped, so it might not be fully comfortable for tall guests; there is an electric kettle, tea, and coffee in the guest room and we used it a few times a day; a bed is quite hard but was fine for us. A private bathroom is clean and minimalistic but has everything you need. There is no AC but no idea whether Vienna is a hot place in summer. The heating was working fine and the room was warm enough.
Breakfast offered by O&V was basically a local bakery, tea/coffee, ham, butter, jam, milk, a banana, and orange juice. For us, it was enough and a good option on the very first morning in Vienna since we didn't know places around. We prefer different food, so on other days, we used to go to "Ulrich" restaurant nearby.
The hosts are very kind and attentive and seem to be in love with their city :). I encourage you to visit O&V!