Certainly, it is spelled correctly in the 'worldofquotes' database: Douglass quotes . I agree with rolig: the general level of the other quotes by Douglass, as well as his background, strongly contradict the notion that he would misspell 'oftenest'.
Yes, but unfortunately, that source does not reference the Douglass quote. I may be wrong, but it seems strange that Douglass would misspell the word this way. If anything, educated former slaves, especially when they were public figures, tended to be overcareful about things like spelling and grammar. The last thing they wanted was to appear illiterate. The word is, of course, properly spelled "oftenest".