When one draws anything that is even slightly complex, or one requires great-looking diagrams, using a great tool such as OG, there are a few basic Methods that need to be learned. One might say, before building a house or software, one has to build tools.
Semi-transparent colours are definitely more attractive. This is a fairly common requirement, so I will give it detailed treatment. Most of my diagrams are complex architecture-type diagrams: I do this on all my Shapes, regardless of how I am going to use them (the diagrams are subject to updates).
Handling a Semi-Transparent Shape
We need to make a a backing for such a Shape. Study the overlapped areas in each step, carefully. The background is set to LightGrey 25%, in order to emphasise the problem.
Once you finalise such a Shape, put it in a Stencil. It is easy to change a rectangle to a circle, etc.