Speaking to British TV show 'Daybreak', she said: "We don't see each other often because we both have our own lives and things, but it's like picking up where we left off, so we've stayed in touch over the years.
"We always have a great rapport with each other and it's always very comfortable. We got together to go over the songs and it was like nothing had ever changed."
'This Christmas' features a host of festive classics such as 'White Christmas', 'Deck the Halls' and 'Baby, It's Cold Outside' and Olivia says John really helped her get to grip with the tracks in the recording studio.
She added: "He sounds great on those songs and loves them and knows them backwards and I felt very comfortable with it."
The two stars are donating all artist proceeds from the album equally to their respective charities, the Jett Travolta Foundation for children's disabilities and the Olivia Newton-John Cancer and Wellness Centre.