Oh, my giddy aunt

"Oh, my giddy aunt" was a colloquialism that was often used as an exclamation of surprise.

By the Doctor
The Second Doctor used the phrase often. He said it to himself when surprised by a Kroton, (TV: The Krotons) to Sergeant Benton as a Gell guard tried to enter the TARDIS, (TV: The Three Doctors) and to Dastari during an argument in a cellar. (TV: The Two Doctors)

A clone of the Second Doctor said it as Lucius threatened to devour him. (AUDIO: Survivors in Space)

The Eleventh Doctor said it while facing a Diceman with Henry Gordon Jago. (AUDIO: The Jago & Litefoot Revival)

By others
In the 1890s, Henry Gordon Jago said it when he saw one of the Wet Men. (AUDIO: Dead Men's Tales)

The Brigadier said this when he found out that Professor Rubeish was missing. (TV: The Time Warrior)

Mrs. Wibbsey exclaimed it to Buddy Hudson and a cultist when realising something was wrong with the Fourth Doctor. (AUDIO: Starfall)

Missy said it to Osgood while they were on a plane. (TV: Death in Heaven)