Here's how to integrate your OptiMonk account on your Opencart site.
1. Login to your Opencart Admin dashboard.
2. Go to the Extensions ->Extensions page and Install Google Analytics
3. Click on the Edit button and insert the OptiMonk code into the Google Analytics Code section and change the status to enabled then save it.
Congratulations, you're all set! Please note, if you would like to use cart segmentation with Opencart, then you have to develop it. More information here.