Joe Kozuh asked a question yesterday that puzzles many more than him: “What is the difference between -er and -or on nouns and is there a rule of thumb that governs their selectioin?”
Generally, -or is a Latin suffix and -er is the Germanic equivalent meaning, roughly, “one who Vs”, where V represents any verb. Words borrowed directly from Latin, then, tend to end on -or: governor, calculator, arbitrator, legislator, alternator. Words of Germanic origin (English is a Germanic language) generally take -er: runner, thinker, worker, joker.
However, two factors muddy the water. English borrowed many words from French in the Middle Ages and the French equivalent of -or and -er, is -eur. English generally reduced that suffix to -er, keeping it only in a few words borrowed late: amateur, restauranteur, raconteur, coiffeur. English also borrowed many verbs from French and added the English suffix: employer, deceiver, certifier.
So, you need to know the etymologies of many of the verbs that -er and -or are added to, in order to know how to distribute them. You can be sure that verbs ending on -ize and -ify will take the suffix -er and that verb ending on the suffix -ate will be suffixed with -or. Other than that, though, we have the etymological rule of a very small and barely helpful thumb.