But a new study says they aren't really related.
Using fMRI brain scans, researchers at the Institute of Living in Hartford, Connecticut, found that the brains of hoarders and those with OCD work differently.
And it explains why therapy used for OCD fails with hoarders.
Volunteers were asked to sort through junk mail while undergoing the brain MRI, to decide what to keep and what to discard.
When the mail had a hoarder's name on it, decision-making areas of the brain lit up among the hoarders. But when another person's name was on it, those areas were abnormally quiet.
The study authors say those results indicate hoarders suffer from decision-making issues, not an obsession with all objects.
They say treatment for hoarding needs to be adjusted, to give them better skills at decision-making and prioritizing.
The study, reported in the Journal of the American Medical Association, comes just as the American Psychiatric Association is debating whether hoarding is distinct disorder, and whether it should be written that way in the next DSM - Diagnostic and Statistical Manual of Mental Disorders. That is the guide used by mental health professionals to diagnose patients.
