When it was originally described by Baillon (1891), Oncotheca was treated as a genus of Aquifoliaceae because of its resemblance to Phelline and Sphenostemon, which were classified within that family prior to being elevated to familial rank (Phellinaceae and Sphenostemonaceae).
For nearly half a century, botanists continued to recognize Oncotheca as a genus of either Aquifoliaceae or Ebenaceae, even though it was considered an isolated taxon by all.
Nevertheless, they concluded that because a close relationship between Oncotheca and either Aquifoliaceae or Ebenaceae could not be justified, only Theaceae remained as the most likely candidate for closest living relative of Oncothecaceae.
Although the exact position of Oncothecaceae among angiosperms is still somewhat unclear, the family shows no affinity to Theaceae, Ebenaceae, Aquifoliaceae, or any other family suggested previously.
But in either case the family appears not to be allied to Aquifoliaceae, Ebenaceae, Theaceae, or any other family previously hypothesized.
Strong aluminum hyperaccumulation is suggested to characterize the genus Phyllonoma (Phyllonomaceae), but it is uncertain whether the feature occurs in the related families Aquifoliaceae and Helwingiaceae (Fig.
Atherospermataceae, Aquifoliaceae, Gomortegaceae, Helwingiaceae, Styracaceae).