In broad terms, on tends to have the slight nuance of specifically; directly; exactly, whereas about tends to have the slight nuance of around; generally; in broader terms; as a whole, although I am not sure that everyone consciously makes this fine distinction.
I don't see much difference, but in practice 'dissertation', 'lecture', 'scholarly article', 'thesis' collocate better with 'on'. It is used for the titles of scholarly works; for example 'De Rerum Natura' is translated as 'On the Nature of Things'.
When you use 'on' [instead of 'about'], you take out the sense of 'freely moving within a particular topic area'. 'on' is used to indicate that the discussion will focus on the specific topic under discussion.
They're equivalent in the sense that you're writing about poetry, or writing on poetry, or writing of poetry: here 'about' is the more common choice. But for titles of essays, we usually choose 'On'.