Oesten's Fantasies Brillantes are designed to impress with virtuosic display and often feature popular melodies of the era. They are characterized by rapid passagework and exciting climaxes.
Focus on developing speed and evenness in rapid scalar and arpeggiated passages.
6 Fantaisies brilliantes (Op.67) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
6 Fantaisies brilliantes (Op.67) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.