SchroCat is applying the general rule in British English, that is when "out" is prefixed to a noun or verb it is usually hyphenated, and is correct in the case of "pre-arrange", which is hyphenated in the OED. However, "outgross" is unique because it is of American origin and adopted into British English; in this instance the OED have adopted the American usage of leaving out the hyphen. If the word was not listed in the OED I think it would be justifiable to apply the general British rule of hyphenating, but since the OED has accepted it in its general American form we should probably follow suit. Kelisi, please remember that different English speaking countries have different varieties of English, and we must respect the form the article is written in per WP:RETAIN.