In　this　study,　boron　powder　with　different　particle　sizes　was　purified　by　both　chemical　and　heat　treatment　methods.　The　reduction　in　the　particle　size　can　improve　the　chemical　purification　with　no　effort　on　the　heat　treatment.　The　superconducting　properties　of　the　powder　drastically　changed　even　with　only　a　partial　elimination　of　oxygen.　On　the　one　hand,　less　oxygen　content　resulted　in　high　T-c　and　J(c)　values　under　the　low　magnetic　field,　and　most　importantly,　a　significant　improvement　in　the　superconducting　connectivity　(A(f)　value).　On　the　other　hand,　the　degradation　of　J(c)　under　a　high　field　and　a　change　in　the　pinning　mechanism　were　also　found,　along　with　decreasing　oxygen.　This　result　indicated　that　oxygen,　probably　MgO,　might　act　as　the　pinning　center　and　as　an　obstacle　for　the　supercurrent　in　MgB2　at　the　same　time.　This　work　paves　the　way　for　obtaining　pure　oxygen-free　MgB2　and　understanding　the　real　effect　of　oxygen　in　MgB2.