This extension lets you accept Bitcoin payments on your OpenCart store with ease. There are no security risks, payments go directly into your own bitcoin wallet. The complete checkout process happens within your website; customers never leave your site. All major HD wallets are supported, as well as all major fiat currencies. No approvals of API key are required, installation is fast and simple.
Supported Worldwide - no restrictions or documentation required.

Feature list:

Add Bitcoin as a payment method on your OpenCart eCommerce store
A "Bitcoin" option is added on customer checkout to Payment Method
No middle man in Bitcoin transfer. Payment goes straight to your wallet
No outside gateway. Customer never leaves the store
Fast and easy setup. No waiting for API approvals
Automatic transaction confirmation monitoring. The order is processed after two confirmations
Monitor expected/paid amounts of BTC
Use any currency selected in your store
BTC amount is always calculated with latest exchange rate

Installation
Here is a blog post with detailed installation instructions

Download 'blockonomics-opencart.ocmod.zip' file
Open OpenCart admin page
Navigate to Extensions -> Installer and upload the zip file
Navigate to Extensions -> Extensions and select type "Payments" and install Blockonomics extension
Proceed to configuration

Configuration

Go to blockonomics.co > Merchants and complete merchant wizard
Get API key from blockonomics.co > Wallet Watcher > Settings and paste it in OpenCart Blockonomics module settings
Click on Test Setup in OpenCart Blockonomics module settings