Convert OOP modules into a DLL

Don't know if you really want to distribute them as packages. Packages can be very, very version dependant. It will solve almost all possible conflicts between application and DLL but whomever else is going to use it must be using almost the same Delphi version including the same patches.

I think you'd prefer it if your DLL is more independant...

My personal preference with objects in DLL's is that I write an Interface definition for it and I just export a function that generates an object that matches the interface. Something like this:

library DLL;
uses Windows;
type
  IMyInterface = interface
    ['{BEB0CA0F-4C43-42F9-9DEE-63F1D6556EED}']
    procedure ShowMessage(const Message: string);
  end;
  TMyInterface = class(TInterfacedObject, IMyInterface)
  protected
    procedure ShowMessage(const Message: string);
  end;

function GetMyInterface: IMyInterface;
begin
  Result := TMyInterface.Create;
end;
exports GetMyInterface;

procedure TMyInterface.ShowMessage(const Message: string);
begin
  MessageBox(GetDesktopWindow, PChar(Message), 'DLL Message', MB_OK)
end;

end.

Yeah, it's overly simplified but it works. The drawback however is that the calling application also needs to know the interface definition so IMyInterface is better defined in a separate unit which then has to be shared.
The advantage however is that interfaces are much less dependant on the right Delphi version. Furthermore, they are less likely to cause memory leaks or memory allocation problems. Most of such problems can occur when an object is created in the DLL but freed in the main application. With an interface, the 'Free' method is always called in the context of the DLL. Unfortunately you still have to be careful when sharing string or array values between your application and DLL since these can still cause memory conflicts because they're cleaned up in the wrong context. (In the sample above, I avoided this by making the string parameter a Const, thus the DLL method knows it's not going to free it.

Of course, there is another solution for this all. You could create a COM wrapper around your objects. (Welcome to the COM Hell in that case.) COM is a bit more complicated than my 'interface' but it would allow you to create your DLL as just one binary with no additional sourcefiles. You'd also allow developers who use other languages (VB/C++) to use your objects which could make it a bit more popular.
Yet COM is a complex technology for beginning developers with a steep learning curve and can cause plenty of problems if your code has some flaws. (And code always has flaws...)