I disregarded the wind and wave loads to check the overturning moment at the tower base (since the CoG of the RNA mass is in front of the tower axis).
Here you can see the longitudinal bending moment at the tower base:

I assume the moment is oscillating since the gravity is internally ramped up, right?
But why is the deflection of the first moment positive? That would mean that the structure bends backwards first…

I found the total mass here -

but I’m not sure where to find the “RNA center of mass relative to the nacelle attachment point on the tower”…

I know it’s a basic question, maybe another beginner user can give me a hint

OpenFAST does not ramp up gravity at the start of the simulation. Rather, the transient dynamic response you are seeing is the start-up transient associated with the initial condition solution, which will depend on what initial conditions you have set (like initial rotor speed and initial blade and tower deflections).

ElastoDyn does not directly compute the center of mass of the RNA. But you can calculate that through an OpenFAST linearization analysis, e.g., as discussed in the following forum topics: