You can show your categories in multi-level as an OpenCart module.
Subcategories will be shown on mouse hover.
Demo : http://demos.soft-php.com/joocart-extensions/

Installation

1. Go to o your store admin panel. Navigate to Extensions -> Extension installer.
2. Click on the upload button. A dialog box should open.
3. Locate the installation file of this extension(category_multilevel.ocmod.zip) and select it.
4. Now this module should be visible in Extensions -> Modules. After locating it in the Module list just click the install button (“ + ” sign) Then edit this installed module and configure it as you want.
5. Go to Extensions -> Modifications and click the Refresh sign at the upper right corner of the page.
6. After you have set up the module correctly, you can assign it to a specific layout from System > Design > Layouts. This is done by choosing a layout and adding the module. After you are done , You need to click Save.