Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source–drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec⁻¹, high I_on/I_off ratios of 10⁵–10⁷, contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1–4.6 cm² V⁻¹ s⁻¹, and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.