What’s the rule to decide whether you add -er or whether you add -or when creating a noun from a verb?
Sometimes it’s -er:
- read > reader
- hate > hater
- hit > hitter
But other times it’s -or:
- meditate > meditator
- collect > collector
This has been previously touched on here: “Commentor” vs “Commentator”. I'll try to expand on that.
Both -er and -or are so-called "agent noun suffixes", and Wiktionary has entries for both:
Generally speaking, -er is much more common in English (which should come as no surprise since it has deep Germanic roots, see the link) and can be easily attached to any English verb to form the corresponding noun (drive — driver, run — runner, drink — drinker, etc.).
The suffix -or, on the other hand, comes from Latin, and is used much more seldom, basically where Latin would do it. Just try building the words drivor, runnor or drinkor, and see for yourself. In fact, Wiktionary lists only a handful of terms that were derived using this suffix, such as actor, author and sculptor, and goes on to provide the following usage notes:
Edit: courtesy of Martha, here's a link to a post on the "Separated by a common language" blog that provides further details and addresses the differences between US and UK English: