Hey, was Leo peeking at his hand? How did he know about the Colonies?
Two turns ago, Leo used a Spy leaving a Colony on top, then Ghost Ship. His opponent had Colony/Colony/Colony/Herald/Ambassador, so he put the Herald and the Ambassador back on top to potentially improve his next turn. Then Leo played an Oracle, but he left those two cards there because they are relatively useless in a deck full of valuable green cards. Finally, Leo played Kings Court->Bureaucrat, forcing his opponent to place the three Colonies on his deck. Therefore, Leo now knows that in two turns' time, his opponent will have the same Colony/Colony/Colony/Herald/Ambassador hand, and the first card the Herald draws will be a Colony.
I am sure that some variation of this leaves an Oasis in his opponent's hand to discard one of the Colonies first.