i think its about 2-3 hours away on the dolmus, my mother travelled last year from marmaris to olu deniz and im sure she said this, the dolmus went to fethiye and they got a taxi from there to olu deniz, you might be able to change and get another dolmus but i think there was a long wait hence them getting a taxi, im sure someone else will have more details about this and will help you

Hi Claire,
Forget the Dolmus, get the autogar ( coach ) from Marmaris to Fethiye, takes about 2.5 hrs to 3 hrs, first class travel, and cheap. Dead simple, find the coach station, tell them when you would like to go, they will give you departure times etc, buy tickets and just turn up at arranged times and go.
Then get the Dolmus up to Hisaronu, takes about 10 mins from Fehtiye
Kev

When we were in Hisaranou a couple of weeks ago, we were going to get the coach from Hisaranou to Marmaris but it takes 3 hours and we have a toddler so decided against it but most of the excursion places run them

I wouldn't reccmomend booking your coach with a tour agency. This is a coach excursion taking you round all the tourist areas of Fethiye and Hisaronu. I have been and it was bad.
To take a coach you want to go to the pamukkale office on the side of tansas. Coach tickets are very cheap and the travel and service is excellent. Make sure you book a return as all tickets are sold one-way!