Based　on　the　density　functional　theory　(DFT),　the　electronic　properties　of　O-doped　pure　and　sulfur　vacancy-defect　monolayer　WS2　are　investigated　by　using　the　first-principles　method.　For　the　O-doped　pure　monolayer　WS2,　four　sizes　(2　x　2　x　1,　3　x　3　x　1,　4　x　4　x　1　and　5　x　5　x　1)　of　supercell　are　discussed　to　probe　the　effects　of　O　doping　concentration　on　the　electronic　structure.　For　the　2　x　2　x　1　supercell　with　12.5%　O　doping　concentration,　the　band　gap　of　O-doped　pure　WS2　is　reduced　by　8.9%　displaying　an　indirect　band　gap.　The　band　gaps　in　3　x　3　x　1　and　4　x　4　x　1　supercells　are　both　opened　to　some　extent,　respectively,　for　5.55%　and　3.13%　O　doping　concentrations,　while　the　band　gap　in　5　x　5　x　1　supercell　with　2.0%　O　doping　concentration　is　quite　close　to　that　of　the　pure　monolayer　WS2.　Then,　two　typical　point　defects,　including　sulfur　single-vacancy　(V-S)　and　sulfur　divacancy　(V-2S),　are　introduced　to　probe　the　influences　of　O　doping　on　the　electronic　properties　of　WS2　monolayers.　The　observations　from　DFT　calculations　show　that　O　doping　can　broaden　the　band　gap　of　monolayer　WS2　with　V-S　defect　to　a　certain　degree,　but　weaken　the　band　gap　of　monolayer　WS2　with　V-2S　defect.　Doping　O　element　into　either　pure　or　sulfur　vacancy-defect　monolayer　WS2　cannot　change　their　band　gaps　significantly,　however,　it　still　can　be　regarded　as　a　potential　method　to　slightly　tune　the　electronic　properties　of　monolayer　WS2.