You have to use

o.removeChild(o.firstElementChild);

in your del() function.

So this is the updated code:

<ol id="table">
  <li onclick="move(this)">1</li>
  <li onclick="move(this)">2</li>
  <li onclick="move(this)">3</li>
  <li onclick="move(this)">4</li>
  <li onclick="move(this)">5</li>
  <li onclick="move(this)">6</li>
</ol>
<button onclick="add()">Append</button> <button onclick="del()">Del</button>
<script>
  function move(o) {
    var parent = o.parentNode;
    parent.removeChild(o);
    parent.insertBefore(o, parent.firstChild);
  }
  function add() {
    var node = document.createElement("li");
    var textnode = document.createTextNode("new");
    node.appendChild(textnode);
    node.setAttribute("onclick", "move(this)");
    document.getElementById("table").appendChild(node);
    id = "myList";
  }
  function del() {
    o = document.getElementById("table");
    o.removeChild(o.firstElementChild);
  }
</script>

The firstChild is not an LI but a textnode

Use

 o.removeChild(o.firstElementChild);

Also delegate the click

const table = document.getElementById("table")
table.addEventListener("click",function(e) {
  const tgt = e.target;
  if (tgt.tagName==="LI") {
    move(tgt);
  }
})
function move(o) {
  var parent = o.parentNode;
  parent.removeChild(o);
  parent.insertBefore(o, parent.firstElementChild);
}

function add() {
  var node = document.createElement('li');
  var textnode = document.createTextNode("new");
  node.appendChild(textnode);
  table.appendChild(node);
}

function del() { 
  table.removeChild(table.firstElementChild);
}

<ol id='table'>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
  <li>6</li>
</ol>
<button onclick="add()">Append</button> <button onclick="del()">Del</button>