Hello, we are at the Old Observatory Hostel and want to go to Laoshan tomorrow the full day.

I hope my answer is not too late for you, Hugo. You may take a taxi to Zhanqiao bus stop within 10min. There you can take bus no. 311 to Laoshan directly within an hour.

Thank you for your quick answer !!
Actually we took a trolley bus to another station and then went to Laoshan by bus (I don't remember the numbers sorry).
It was fine but for those read me here, be aware that the return buses are very VERY crowded so don't wait for the last one (around 6 PM) and go back earlier, like before 5 PM