We present observations with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope of five star-forming galaxies at redshifts in the range 0.2993–0.4317 and with high emission-line flux ratios O₃₂ = [O iii] λらむだ5007/[O ii] λらむだ3727 ∼ 8–27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions f_esc(LyC) in a range of 2–72 per cent. A narrow Lyαあるふぁ emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks V_sep varying from ∼153 to ∼345 km s⁻¹. We find a general increase of the LyC escape fraction with increasing O₃₂ and decreasing stellar mass M_⋆, but with a large scatter of f_esc(LyC). A tight anticorrelation is found between f_esc(LyC) and V_sep making V_sep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.