By surface analysis, ota + niemi. However, this is likely a folk-etymological alteration of the Swedish name Otnäs, which may actually derive from an original Finnish name *Hautaniemi (*Houtnäs > *Outnäs > Otnäs), from hauta(“grave”) + niemi, and there is indeed a barrow in the area.[1]
The internal locative cases (inessive, illative and elative) are used with this place name when referring to a location; for example, "in Otaniemi" is Otaniemessä.
^ Sirkka Paikkala, editor (2007), Suomalainen paikannimikirja [Book of Finnish Place names] (in Finnish), Helsinki: Karttakeskus, Kotimaisten kielten tutkimuskeskus, →ISBN