IPA: /ˈɔːftəst/
KK: /ɔːftɛst/
This word means the most frequent or common occurrence of something.
She visits her grandmother oftentimes, but today she is the oftest visitor this week.
Oftest is derived from "oft" (from Old English "oft", meaning frequently or often) and the superlative suffix "-est" (meaning most). The word "oftest" means most frequently or most often.
Think of 'oft' meaning frequently, and when you add '-est', it indicates the highest degree. So, oftentimes means the most frequent occurrence.