Javascript, 75

function o(s){return s+((0|s/10%10)==1?"th":[,"st","nd","rd"][s%10]||"th")}

Javascript, (削除) 75 (削除ここまで) 60

With the new arrow notation:

o=(s)=>s+((0|s/10%10)==1?"th":[,"st","nd","rd"][s%10]||"th")

Old version, without arrow notation (75 chars):

function o(s){return s+((0|s/10%10)==1?"th":[,"st","nd","rd"][s%10]||"th")}

#Javascript, 75 function o(s){return s+((0|s/10%10)==1?"th":[,"st","nd","rd"][s%10]||"th")}

Javascript, 75

function o(s){return s+((0|s/10%10)==1?"th":[,"st","nd","rd"][s%10]||"th")}