Since OPC 2.0.285 we dropped support for smaller than 2mb upload size and for this reason, we are providing the Installation guide on how to install (any) extension above 2 megabytes in case your hosting provider does not allow uploads larger than the extension that you'd like to install.
Guide 1, with RuposTel Unzip Utility for Joomla
2. Download OPC package (for example onepage2.0.283.24022015.zip)
This guide will also work with Virtuemart installation files or any other zip, tar.gz or other archives
3. Upload both unzip.php and the zip archive to your Joomla's /tmp directory as shown on this image:
4. Visit URL of http://yoursitecom/tmp/unzip.php
5. Select package: