We propose a scheme for an in-line spin-torque nano-oscillator (iSTNO) composed of a single nanowire. The oscillating element is an in-plane magnetized region of an otherwise out-of-plane magnetized nanowire, which supports a spin-wave mode. Analytical exploration reveals that the nonadiabatic spin-transfer torque can cancel out the damping and thus induce sustained precession in response to a direct current. Moreover, it predicts that the frequency scales linearly with current and that the wave vector depends only on geometry. Simulations with single iSTNO cells confirm that oscillations occur, and simulations with two iSTNO cells show that they frequency lock to each other roughly in antiphase, even with mismatches in geometry. iSTNO devices can be easily fabricated by irradiating regions of a perpendicular magnetic anisotropy nanowire with ions, do not require an external field, inherently support multiple iSTNOs, and thus form an attractive alternative STNO.