It’s neither vorOnoi or vOronoi. It’s VoronOi, as in Georgy Voronoy, the Russian mathematician, who defined the Voronoi diagram.
That last “I” is also questionable, cause there’s a universal rule, where Russian letter “Й” should be transliterated to “Y” and not to an “I”.
Someone had to say it. Also, voronoi is a horse color and a color that we get after bluing steel.