Doctor Who Series 10: 11 Easter Eggs & References In 'Oxygen'
9. Leonardo
When in the TARDIS, Bill can be seen reading a book titled Leonardo. No, she isn't reading about one of the Teenage Mutant Ninja Turtles but renaissance artist Leonardo Da Vinci.
The Doctor is an old friend of Leo's, as was first established in 1979's City of Death and reiterated in 1996's Doctor Who: The Movie (the Eighth Doctor tells Grace "he had a cold when he drew that" after seeing a Da Vinci sketch).
The back cover of the book depicts Da Vinci's most famous work, the Mona Lisa. The painting has been the subject of two stories of the Whoniverse - the aforementioned City of Death and The Sarah Jane Adventures' episode Mona Lisa's Revenge.