From OpenCart Admin Dashboard, select Extensions → Installer. Select the onWebChat extension (.zip file) and upload it.
Select Extensions → Extensions and from drop down menu, select Modules. Find onWebChat extension and click on "+" button. Then click on pencil button.
Paste your Chat Id, select "Enable" and click on "Save".
If you don't have an onWebChat Chat Id, please create an account here.
Go to 'Design' → 'Layouts' section to set the appearance of live chat widget on your website layouts.(ex. Home, categories, products..). You should do it for all layouts in order to show the live chat widget on all your webpages. Click on "edit" icon of layout.
Click on "+" button. Then click on "edit" and select "onWebChat - Live Chat" from Content Bottom. Final click on "save" button.
Once you have saved your changes, you should see onWebChat - Live Chat widget on your OpenCart store.