Ondrej is a very nice and welcoming host. His flat is located on the ground floor of a typical Prague's building. The flat is nice and good-looking, with two big bedrooms and a cozy living room. The bathroom is a little bit small and with few shelves, but it was definitely ok for our short stay.
The flat is approximately 10 minutes walk from the Old Town and 5 minutes walk from Muzeum subway station where you can also find a supermarket (Albert's).
In case you need a lift to the airport, you can ask Ondrej and he will drive/catch you to the airport for 700 Czech Crowns (FYI Prague's taxi drivers have a nasty reputation so they would probably ask you for a higher amount).
If I ever come back to Prague, I will definitely look for staying at Ondrej&Nicole's flat.