With a maximum total length (including tail) of ,
F. oustaleti is generally considered the world's largest species of chameleon by this measurement. Exceptionally large
Calumma parsonii (Parson's chameleon) and
Trioceros melleri (Meller's chameleon) have been claimed to reach even greater lengths, but this remains unverified.
F. oustaleti is relatively slender and its weight is surpassed by certain other large chameleon species that are more robust, notably
C. parsonii. Large adult males of
F. oustaleti typically weigh , but lower weights are common and females consistently weigh less. Females also remain considerably smaller than males in total length, reaching up to about . The head bears a high casque with several crests, and a dorsal crest consisting of 45 or more small triangular spines, runs along the spine. The colouring of
F. oustaleti is quite variable. Males are usually largely grey or brownish, sometimes with reddish-orange feet or underparts. Females are more variable and often more colourful than males, with base colour or markings in red, yellow and green being possible, and the eyelids can sometimes be blue. Oustalet's chameleon (Furcifer oustaleti) male Anja Community Reserve.jpg|male,
Anja Community Reserve Oustalet's chameleon (Furcifer oustaleti) juvenile Montagne d’Ambre.jpg|juvenile male,
Montagne d'Ambre National Park Furcifer oustaleti 186573454.jpg|female,
Ankarana Special Reserve Oustalet's Chameleon (Furcifer oustaleti), Ankarafantsika, Madagascar (4315268487).jpg|female,
Ankarafantsika National Park Oustalet's chameleon (Furcifer oustaleti) young female Isalo.jpg|young female,
Isalo National Park ==Diet==