You cannot place executable files (.so, .dex, .jar) into OBB (expansion files). This is not a technical limitation, it is a policy one, and for good reason.
Expansion files are stored on untrusted storage (for example, sometimes on an SD card). A malicious app might be able to alter the expansion files and change your code after it was downloaded. For this reason, it is unsafe to put any executable code in an expansion file, and so Google Play policy does not allow it.
You may want to look at dynamic feature modules as one way of reducing your app size through delivery of dynamic code.