We present a spectroscopic study of metal-deficient dwarf galaxy candidates, selected from the SDSS DR12. The oxygen abundances were derived using the direct method in galaxies with the electron temperature-sensitive emission line [OIII]4363A measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [OIII]4363A line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 low-metallicity candidates with oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient (XMD) candidates with 12+log O/H<7.35. Ten out of sixteen XMDs known so far (or ~60%) have been discovered by our team using the direct method. Three XMDs were found in the present study. We study relations between global parameters of low-metallicity galaxies, including absolute optical magnitudes, Hbeta luminosities (or equivalently star formation rates), stellar masses, mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies strongly deviate to lower metallicities in L-Z, L(Hbeta)-Z and Mstar-Z diagrams than in relations obtained for large samples of low-redshift, star-forming galaxies with non-restricted metallicities. These less chemically evolved galaxies with stellar masses ~10^6-10^8Msun, Hbeta luminosities ~10^38-10^41 erg/s, SFR~0.01-1.0Msun/yr, and sSFR~50 Gyr^-1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.
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