(I'm a beginner in Ogre)
I have an old 3d OpenGL project written in C#,
now i have a need to add some complex object,
is it easy to use Mogre here? where should i begin?
First you should study the beginner tutorials in the wiki.
After that you should know, how Mogre works (which is not that diffucult to understand) and how to set up a scene, ...
When you could try to search for suitable classes and functions for your OpenGL project but because I do not know your project its up to.
And search the forums (also the Ogre forum), there are a lot of questions with very good answers
But keep in mind, that Ogre (Mogre) is desinged to to be a high level graphic engine -> There are no low level functions because they are wrapped into classes and so on.