Dexter's Laboratory Wiki
Grandpa O'Reilly
Gender Male
Species Human
Family Dextamina (sister)
Dexter (grandson)
Dee Dee (granddaughter)
Dad (son-in-law)
Mom (daughter)
Fergle O'Reilly (son)
Unnamed daughter
Interests Science
First Appearance The Grand-Daddy of All Inventions
Voiced By Christine Cavanaugh

Grandpa O'Reilly is Dexter and Dee Dee's maternal grandfather, Dextamina's younger brother, and Mom and Fergle's father. He was voiced by Christine Cavanaugh.


Just like his grandson Dexter, he also has a laboratory, as shown in The Grand Daddy of All Inventions. However unlike Dexter's massive and high tech lab, his was a small space in his garage. Grandpa is also implied to be smarter than Dexter since, with a relatively simple set up he was able to create a self sustaining power source, which Dexter failed to do with his complex machinery.

Episode Appearances[]


  • He is Mom's father since Mom refers him as "Dad".
  • His design is identical to Dexter's future self, Old Man Dexter, indicating that Dexter inherits both his looks and genius from his grandfather.
  • His only official appearance was in the episode The Grand Daddy of All Inventions however, he was mentioned in the episode Old Man Dexter.
  • It is unknown what type of relationship he has with his son, sister, and other daughter.