Overall: We had a very pleasant stay at Oktay's accommodation, and the place looked just like in the photos. It's also quite near to Istiklal Street and relatively easy to find via public transport. Overall, notwithstanding some not too major cons, we've rated this 5 stars
Pros: Oktay and his mum were very welcoming, even though our check-in was late in the night. Oktay was responsive to our queries throughout our stay, replying our messages almost immediately 90% of the time. The accommodation was spacious and warm (we stayed in mid-March when it's cold) for two people, and relatively clean.
Cons: Some steep slopes to climb from the tram station (much of Istanbul is hilly anyway) and the street to the apartment is narrow so taxis might not want to travel all the way up, and cobbled too, so something to take note of with heavy luggage.