Often
Middle English: extended form of oft, probably influenced by selden ‘seldom’. Early examples appear to be northern English; the word became general in the 16th century.
wiktionary
From Middle English often, alteration (with final -n added due to analogy with Middle English selden(“seldom”)) of Middle English ofte, oft, from Old English oft(“oft; often”), from Proto-Germanic *ufta, *uftō(“often”). Cognate with Scots oftin(“often”), North Frisian oftem(“often”), Saterland Frisian oafte(“often”), German oft(“often”), Norwegian and Danish ofte(“often”), Swedish ofta(“often”), Icelandic oft(“often”).
etymonline
often (adv.)
"repeatedly, again and again, many times, under many circumstances," mid-13c., an extended form of oft, in Middle English typically before vowels and h-, probably by influence of its opposite, seldom (Middle English selden). In common use from 16c., replacing oft. Related: Oftener; oftenest.
