I would be very surprised to find that it's not possible to have a drop down menu showing applications that may be launched, instead of having individual buttons on the application toolbar. I tried to find a way to do this, but I must admit that my search did not produce any result. I therefore turn to you for assistance. Is it indeed possible to have this sort of structure?
Yes, just go into Settings -> Customize mode, right-click some empty space on the toolbar and choose New -> New Menu, then click the menu you just created to open it up, after which you can drag whatever you want into it from your toolbars (or from shortcuts etc. outside of Opus).
Thank you very much. I'm delighted to see that it works as advertised. 
Would you please explain what the New Menu Button does? It looks very similar to the Menu button and I don't get what the dropdown arrow on it is supposed to do.
Menu Buttons combine a button with a menu. Like the Copy Files, Move, Rename, New Folder, Delete etc. buttons on the Opus 10 default toolbars.
Thank you for the explanation.