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V magistrskem delu je raziskana kemijska homogenost Orionovega kompleksa molekularnih oblakov na podlagi meritev zastopanosti kisika v posameznih zvezdah. Najprej so predstavljene osnove zvezdne spektroskopije. Opisana sta spekter črnega telesa in formacija absorpcijskih črt. Podrobneje je opisan teoretični Voightov profil absorpcijskih črt z upoštevanjem tlačne in termične razširitve, opisani pa so tudi premiki in razširitve črt, ki izvirajo iz zvezdne dinamike. Predstavljena je tehnika Echelle spektroskopije, HR in Kielov diagram ter spektralna klasifikacija zvezd na podlagi podobnosti v njihovih spektrih. Sledi opis procesov življenja zvezd -- od njihove formacije iz molekularnih oblakov in obdobja na glavni veji, do razvoja v orjakinje ter smrti zvezd in obogatitve okoliškega prostora z elementi nukleosinteze. Predstavljene so tudi glavne fizične in kemijske značilnosti Orionovega kompleksa. V praktičnem delu naloge so uporabljena spektroskopska opazovanja 30 zvezd iz območij molekularnih oblakov v Orionu in Samorogu v območju valovnih dolžin od 395 do 680,nm. V glavnem so to vroče zvezde glavne veje, nekaj pa je tudi orjakinj. Opisan je postopek obdelave surovih spektrov v programu IRAF, za katere sem nato z uporabo programa iSpec določil vrednosti radialne hitrosti, atmosferskih parametrov (efektivna temperatura, gravitacijski pospešek na površju, mikro- in makroturbulenčna hitrost, projekcija rotacijske hitrosti in koeficient robne zatemnitve) in zastopanosti kisika. Zastopanosti kisika sem določil pod predpostavko kemijske homogenosti kompleksa v kovinskosti in zastopanosti $alpha$ elementov -- pričakoval sem torej enakomerno zastopanost kisika v različnih spektralnih tipih zvezd. Območji sta glede na zastopanosti kisika v zvezdah homogeni. Določena absolutna zastopanost kisika v opazovanih zvezdah znaša $A$(O)$=(8,33 pm 0,49),$dex, določena relativna zastopanost glede na železo pa [O/Fe]$=(-0,26pm0,49),$dex. Povprečna velikost napake določene zastopanosti kisika vseh zvezd je enaka $0,54,$dex. Raziskani so tudi vplivi posameznih parametrov na končne določene zastopanosti kisika. Med vsemi parametri na končno napako najbolj vplivata natančnost določitve efektivne temperature in gravitacijskega pospeška na površju. The Master’s thesis explores the chemical homogeneity of the stars of the Orion Molecular Complex, based on measurements of Oxygen abundance in individual stars. Stellar spectroscopy is introduced, followed by the description of a blackbody spectrum and the formation of absorption lines. Theoretical Voigt profile of absorption lines is described in detail by taking pressure and thermal broadening into account. Doppler shift and rotational broadening of lines that originate from the star dynamics are described as well. Furthermore, the Echelle Spectroscopy technique, HR and Kiel diagram, and the spectral classification of stars, based on similarities in their spectra, are presented. The process of the stellar life is described from the formation in molecular clouds and the main sequence phase, to the evolution into giants and finally deaths that enrich the surrounding environment with the elements of stellar nucleosynthesis. Theoretical part is concluded with the presentation of physical and chemical features of the Orion Complex. In practical part, spectroscopical observations of 30 stars from the molecular cloud areas in Orion and Monoceros, covering the area of wavelengths from 395 to 680 nm, have been used. Mainly, these are hot main sequence stars, including a few giants. It is described how raw spectra have been processed in IRAF programme and how iSpec programme has been used to determine the radial velocities, atmospheric parameters (effective temperature, gravitational acceleration on the surface, micro- and macro-turbulence velocities, the projection of the rotational velocity and the limb darkening coefficient) and most importantly, the Oxygen abundances of observed stars. Oxygen abundance have been determined under the assumption that Orion complex is chemically homogeneous in metallicities and abundances of $alpha$ elements – therefore, the Oxygen abundance should not vary between different spectral types. Considering Oxygen abundance, the Orion Complex is proven homogenous. The determined absolute Oxygen abundance in the observed stars is $A$(O)$=(8,33pm0,49),$dex, while the determined abundance, relative to iron is [O/Fe]$=(-0,26pm0,49),$dex. The average spread of determined Oxygen abundances of all stars is equal to 0,54,dex. Lastly, the relations between the determined Oxygen abundances and uncertainties of individual parameters have been researched. Among all the parameters, what affects the Oxygen abundances the most, is the accuracy of determining the effective temperature and the gravitational acceleration on the surface.