cyru5 welcome to the QR forum. You will hopefully find the Qr dashboard a very useful tool.
Since you are in the J (business) cabin on QR or OW (Oneworld) flights you automatically have access to Al Mourjan.
To understand what is happening you need to understand the lounge structure at DOH
1) Al Safwa is the premiun first class lounge for F passengers on QR or OW flights.
2) Al Mourjan is the premium Business class lounge for J passengers on QR or OW flights
Several levels of quality and ambience below these are
3) First class lounge which is mainly for QR Platinum and OW Emerald status passengers
4) Business class lounge which is mainly for QR Gold and Silver and OW Sapphire status passengers.
There is also the paid Oryx lounge. But ignore that for the moment. It will not figure here.
Your travel class decides which of the lounges you have access to for 1 and 2 on the list above and your status level with Privikege Club/Oneworld 3 and 4.
Once your lounge access has been decided you also have access to lounges lower down the list.
This is useful if you are in J (Al Mourjan), you have status and you wish to invite guests into a lounge then 3 and 4 can be used..
While it is an exaggeration you could say that 1 and 2 are five star and 3 and 4 are two star lounges
Access rules are slightly more complicated than indicated here but you can see them in the Wiki on the
QR Lounges at HIA dashboard widget.