These words sound the same, but they’re not. A callus is a rough patch of skin. Add an "o" for "offensive" and you get callous, an adjective meaning "insensitive to the feelings of others."

A callus is a place on the skin that’s thick from rubbing up against something. Calluses can be annoying, but they're helpful for some activities. If you’re learning to play guitar, for example, calluses on your fingertips make it easier to press the strings down. Here are some rough patches in the wild:

If you develop a corn or callus, your doctor can help you safely remove the extra layers of skin. (Washington Post)

Most telling are the smoothed calluses on his fingertips; one of his many jobs is playing the bass guitar for his local church. (The Guardian)

Being callous means like acting like a callus — hard and insensitive. Ignoring someone’s plea for help would be a callous thing to do. Here are some hard-hearted examples:

The piece portrayed Woods as a callous cheapskate who enjoyed firing members of his entourage. (Reuters)

I was so often callous to the feelings of others and so selfish. (Fox News)
Just to make things more confusing, if you have a callus on your hand, your hand is calloused. To help keep things straight, remember that callous and calloused are always adjectives. Here’s an example:
If San Francisco is white gloves and executive suites, Oakland sees itself as calloused hands and backs bent in honest labor. (Los Angeles Times)
The words share a Latin root meaning "hardened,” but callus is a noun and callous is an adjective. A callus (without an o) only happens to u, and callous has an o for offending others.