An O-type star is a hot, blue star of spectral type O in the Yerkes classification system employed by astronomers. They have surface temperatures in excess of 30,000 kelvins (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.

Stars of this type are very rare, but because they are very bright, they can be seen at great distances; out of the 90 brightest stars as seen from Earth, 4 are type O. Due to their high mass, O-type stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Most of these stars are young massive main sequence, giant, or supergiant stars, but also some central stars of planetary nebulae, old low-mass stars near the end of their lives, which typically have O-like spectra.

O-type stars are typically found in regions of active star formation, such as the spiral arms of a spiral galaxy or a pair of galaxies undergoing collision and merger (such as the Antennae Galaxies). These stars illuminate any surrounding material and are largely responsible for the distinct bluish-white and pink coloration of a galaxy's arms. Furthermore, O-type stars often occur in multiple star systems, where their evolution is more difficult to predict due to mass transfer and the possibility of component stars exploding as supernovae at different times.

O-type stars are classified by the relative strength of certain spectral lines. The key lines are the prominent He⁺ lines at 454.1 nm and 420.0 nm, which vary from very weak at O9.5 to very strong in O2–O7, and the He⁰ lines at 447.1 nm and 402.6 nm, which vary from absent in O2/3 to prominent in O9.5. The O7 class is defined where the 454.1 nanometer He⁺ and 447.1 nanometer He⁰ lines have equal strength. The very hottest O-type stars have such weak neutral He lines that they must be separated on the relative strength of the N²⁺ and N³⁺ lines.

The luminosity classes of O-type stars are assigned on the relative strengths of the He⁺ emission lines and certain ionised nitrogen and silicon lines. These are indicated by the "f" suffix on the spectral type, with "f" alone indicating N²⁺ and He⁺ emission, "(f)" meaning the He emission is weak or absent, "((f))" meaning the N emission is weak or absent, "f*" indicates the addition of very strong N³⁺ emission, and "f+" the presence of Si³⁺ emission. Luminosity class V, main-sequence stars, generally have weak or missing emission lines, with giants and supergiants showing increasing emission line strength. At O2–O4, the distinction between main sequence and supergiant stars is narrow, and may not even represent true luminosity or evolutionary differences. At intermediate O5–O8 classes, the distinction between "O((f))" main sequence, "O(f)" giants, and "Of" supergiants is well-defined and represents a definite increase in luminosity. The increasing strength of Si³⁺ emission is also an indicator of increasing luminosity and this is the primary means of assigning luminosity classes to the late O type stars.

Star types O3 to O8 are classified as luminosity class sub-type "Vz" if they have a particularly strong 468.6 nm ionised helium line. The line's presence is thought to indicate extreme youth; the "z" stands for zero-age.

To help with the classification of O-type stars, standard examples are listed for most of the defined types. The following table gives one of the standard stars for each spectral type. In some cases, a standard star has not been defined. For spectral types O2 to O5.5, supergiants are not split into Ia / Iab / Ib sub-types: Subgiant spectral types are not defined for types O2, O2.5, or O3. Bright giant luminosity classes are not defined for stars hotter than O6.

O-type stars are hot and luminous. They have characteristic surface temperatures ranging from 30,000–52,000 K, emit intense ultraviolet ('actinic') light, and so appear in the visible spectrum as bluish-white. Because of their high temperatures the luminosities of main sequence O-type stars range from 10,000 times the Sun to around 1,000,000 times, giants from 100,000 times the Sun to over 1,000,000, and supergiants from about 200,000 times the Sun to several million times, although their masses are no more than about 200M_☉.