There is a research that shows that designers are over-concerned with consistency. While consistency means that users don't have to relearn things (by that increasing usability), consistency for the sake of it isn't exactly what UX is about - weighing all variables involved. Consistency is not everything.
In the case of buttons, the consistency argument completely breaks. A label like 'OK' tells the user nothing of what will happen next if you click on it, as the following mockup demonstrates:
download bmml source – Wireframes created with Balsamiq Mockups
To be able to tell what will happen when they click 'OK', users have to search for clues elsewhere, and that's what the text on modals often provides.
But to increase usability, you should make people read as little as possible. That's exactly what 'Move person' does. If the label isn't clear enough, there longer complementary text to elaborate.
download bmml source