Read the documentation for OverlayVideo where it says "The overlays Subscript[o, i] can be any of the following:", and look at how you can use Placed like this Placed[expr,{pos,opos}]. To be specific, you add another position specification for which point in the overlay should align with the point given for the background. Concretely, use any of these:
Placed[..., {{Left, Bottom}, {Left, Bottom}}]
Placed[..., {{0, 0}, {0, 0}}]
Placed[..., {Scaled[{0, 0}], Scaled[{0, 0}]}]
(At least that's how I'm interpreting your question--play with the specifications to get what you want.)
