Well, this seems an appropriate question for me to answer.

As Jonah mentioned, Ollivander was **"enthralled"** by Voldemort.

> Even now, having been tortured and imprisoned by Voldemort, the idea of the Dark wizard in possession of this wand seemed to enthral him as much as it repulsed him.

>*Deathly Hallows, Chapter 24, The Wandmaker*

I believe that this was a technical admiration as much as anything. It would be the equivalent of someone who designs bombs for a living seeing their work used for terrible destruction by a violent dictator. They'd be repulsed but also perhaps curious on some level to how the weaponry operated in a real world situation. Ollivander, who had a deep interest in theoretical wandlore, would've been fascinated to see how a wizard as powerful as Voldemort would wield a wand as powerful as the Elder Wand. That interest may shape how he views (and names) Voldemort to some extent.

I also agree with Himarm that the torture he suffered would have imbued Ollivander with a **fearful respect** for Voldemort. Consequently, he called him by the name Voldemort wanted the wizarding community to use for him.

>Ollivander looked stricken.  
"He was torturing me!" he gasped. "The Cruciatus Curse...you have no idea..."

>*Deathly Hallows, Chapter 24, The Wandmaker*

Finally, it's not unheard of for **non-Death Eaters** to call Voldemort the Dark Lord. Although Death Eaters do seem to be the main group that uses the term there's no reason why they have to have a monopoly on its usage.  It seems that you could use You-Know-Who, He-Who-Must-Not-Be-Named or The Dark Lord interchangeably without comment. Only using 'Voldemort' itself was unacceptable in mainstream society. Trelawney, for instance, seemed to use it as her default term whenever she made a prophecy.

> *The Dark Lord lies alone and friendless...The Dark Lord will rise again with his servant's aid*.

>*(Prisoner of Azkaban, Chapter 16, Professor Trelawney's Prediction)*