Doctor Who Series 10: 11 Easter Eggs & References In 'Oxygen'
10. TARDIS Fluid Link
In an attempt to keep the Doctor grounded, Nardole removes a fluid link from the TARDIS, something the Doctor previously told him would stop it from taking off.
The TARDIS fluid links were first mentioned all the way back in the show's second every story The Daleks. On that occasion, the Doctor sabotaged his own ship by removing a fluid link and claiming that its mercury content needed replacing. Similar to here, this was a clever - if totally reckless - lie to convince his friends to allow him to explore the Dalek city.
The fluid links were mentioned on and off throughout the First and Second Doctors adventures, as the go-to way of putting the TARDIS out of action. The last time the concept was mentioned was in 1968's The Mind Robber.