Spelling- -or or -er?

[Unregistered]

Is there a rule for spelling words ending - -or or -er?

[Casiopea]

Is there a rule for spelling words ending - -or or -er?

The majority of English words ending in -or and -er were borrowed from Latin, German, and French. If you want to construct a spelling rule, you'd have to know the word's origin, and what -or and -er meant in that language. Otherwise, memorization is your only option.