We present a theoretical study of two-terminal nitride-based oscillators utilizing two different hot-electron transport regimes determined by the interaction of the electrons with polar optical phonons, which are capable to generate current/voltage oscillations in the THz-frequency range. The first is the limited space-charge accumulation (LSA) regime based on the negative differential resistance (NDR) in a bulk-like GaN structure at room temperature at high electric field E > Et (Et ≈ 150 kV/cm). The second is the streaming regime in a quantum well (QW) structure in moderate electric field (1-10 kV/cm) at the nitrogen temperature or higher. The latter corresponds to the optical-phonon transit-time (OPTT) resonance and typically does not lead to an NDR at zero frequency. We show that for both regimes, real part of the electron dynamic mobility can be negative within certain THz-frequency windows, whose location and width depend on E. For a 100-nm n-GaN diode with a cross-section of 500 μm<sup>2</sup> and the electron density of 1×10<sup>17</sup> cm<sup>-3</sup>, the generated microwave power is estimated to be ≈ 0.6 W with the dc-to-rf conversion efficiency ≈ 9 % and the magnitude of the NDR of -1.3 Ω. When the streaming transport is realized in the QW channel, the generated power is estimated to be about 350 mW with the efficiency of few percent for a ten QWs GaN-based structure. Hence, the investigated transport mechanisms provide efficient mean to achieve very high-frequency microwave generation in the nitrides.