C H Gardener correctly states that Meissner, Armstrong, de Forest, Langmuir, and Round almost simultaneously discovered that valves with control grids could made to oscillate. However, according to Hugh G. J. Aitken 'The Continuous Wave: Technology and American Radio, 1900 - 1932' (Princeton, NJ (USA): Princeton University Press, 1985), page 239, Lowenstein also made this discovery at approximately the same time as the other inventors.
Furthermore, according to the same page of Aitken's book, the first man who has been documented to have discovered the oscillating ability of valves with a control element was Robert Hutchins Goddard, the inventor of the liquid-fuelled rocket. Goddard filed a patent for a valve oscillator on 1 August 1912. The U.S. patent number is # 1,159,209. Goddard's valve operated somewhat like a modern cathode ray tube: an electron beam was emitted by the cathode, and an external coil swept the beam between two anodes that were connected across an LC network in push-pull fashion. Goddard used the oscillator to provide RF power for his research into the electrodynamic properties of insulators. See his paper: 'On ponderomotive force upon a dielectric which carries a displacement current in a magnetic field' in the Physical Review, Volume VI, Number 2, pages 99 - 120 (August 1914).
Unfortunately, almost every history of valves neglects Goddard's priority.