He used to use the rather classic "He-Who-Must-Not-Be-Named":
"After all, He-Who-Must-Not-Be-Named did great things -- terrible, yes, but great."
(The Philosopher's Stone, Chapter 5, Diagon Alley)
But then after being freed from captivity in the Malfoy mannor, he uses "the Dark Lord", like Death Eaters (and Snape. Why HE named him so is asked here for those interested.):
“The Dark Lord,” said Ollivander in hushed and frightened tones, “had always been happy with the wand I made him"
“The Dark Lord no longer seeks the Elder Wand only for your destruction, Mr. Potter."
"the idea of the Dark Lord in possession of the Deathstick is, I must admit... formidable.”
(Deathly Hallows, Chapter 24, The Wandmaker)
Why did he change his habit and begin using a term primarily used by Death Eaters, even though he was clearly not friendly toward them?