You're missing ; behind the function:
<script>
<?
echo 'oaktree.addItem("test1", branch1, "");';
echo 'oaktree.addItem("test2", branch1, "");';
?>
</script>
echo does not automatically include a new line. The above would echo as:
oaktree.addItem("test1", branch1, "");oaktree.addItem("test1", branch1, "");
Which is why the ; is mandatory for it to work. You can also include a new line yourself but adding a ; by default is a better approach.
You could have probably spotted this error by looking at the source of the page or opening up the console.