[en] Characteristics of low frequency (LF) oscillations of low density plasma νsub(pi) <= νsub(ci) where νsub(pi) and νsub(ci) are ion plasma and cyclotron frequencies, respectively, have been experimentally investigated. Experiments were carried out in the ''Ogra-3B'' simple mirror. LF-oscillations of 1-500 kHz were recorded by electrostatic antennas (including mobile ones) located near plasma surface. The antennas allowed to measure the mode structure and oscillation amplitude out of plasma. To measure oscillation characteristics in plasma cold ion and electron flux fluctuations behind the mirrors along line of force have been recorded. The experiments carried out show that intensive LF-oscillations may be excited in open traps with minimum B. LF-oscillation generation is accounted for within the frames of a simple model of plasma column natural forced flute oscillations. The waves are excited by injected beam current fluctuations at density below the threshold of high-frequency (HF) oscillation origination and by HF-oscillations at higher densities near cyclotron frequencies