var fbUls =document.getElementById("oUl");
var fbLis = fbUls.getElementsByTagName("li");
var aDivs = document.getElementsByClassName("theDiv");
for(var i = 0,val=fbLis.length;i<val;i++){
 fbLis[i].index = i; //存放当前元素的下标
 fbLis[i].onclick = function(){
  for(var j=0; j< fbLis.length; j++){
   fbLis[j].className="";
   aDivs[j].style.display = "none";
  }
  //给当前点击的元素添加活跃标记
  this.className="liactive";
  //显示对应的模块内容
  aDivs[this.index].style.display = "block";
 };
}

<div>
 <ul id="oUl">
  <li class="liactive">首页</li>
  <li>产品</li>
  <li>合作</li>
 </ul>
</div>
<div class="theDiv" id="oDiv1" style="display:block;">首页模块内容：这是首页</div>
<div class="theDiv" id="oDiv2">产品模块内容：这是产品</div>
<div class="theDiv" id="oDiv3">合作模块内容：这是合作</div>