Why are certain atoms more electronegative than others? for example, what is it about oxygen that makes it more electronegative then the elements to its left on the periodic table?

Electronegetivity is the tendency of none metals to attract electrons from another none metal. and that tendancy varies with electronegetivity.  so oxygen can attract electrons more than others do

Electronegativity is defined as ability of  atom to attract a bond pair of electron to itself and it varies among different atoms 
oxygen is more electronegative then the elements to itsleft on the periodic table because it has greater share of bonding electrons and high power to attract electrons than other
hope it helps