How to avoid wordpress cleaning the code of a post.

Hello,
my problem take place when I write a post similar to this:

<div class="sds">
<ol>
<li>d</li>
<li>fd</li>
</ol>
</div>

After saving it, if i try to edit the code this become:

<p class="sds"></p>
<ol>
<li>d</li>
<li>fd</li>
</ol>

The div become a p and the ol is put out from the element.
Does a method exist to avoid wordpress “cleaning” the code?
I tried to modify the tyniMCE file but it didn’ work 🙁