The oxonium salt triethyloxonium tetrafluoroborate is a well known alkylating reagent. In this compound three ethyl groups are attached to a positively charged oxygen atom. Recently a novel and remarkably simply oxonium salt based on a triquinacene skeleton with again a central oxygen atom was described by Mark Mascal et al (DOI).
This work is a follow-up on research published in 1996 on azatriquinacenes (DOI) and azaquinanes (DOI) which are both the aza analogues. The oxatriquinane compound was found to be especially stable: it can survive boiling water for at least 70 hours. its synthesis starts from a cyclononatriene.
Update: Haley predicts further alkylation of new oxonium ion to R4O2+ species is possible (DOI)