To show a checkbox for the terms and conditions in OneStepCheckout the following steps must be performed:
First enable the terms and conditions in the store view:
- Stores > Configuration - select correct store view.
- Sales > Checkout > Enable terms and conditions > "Yes"
Then, in order to render it as a checkbox and not a link:
- Stores > Terms and conditions
- Find the correct terms for the store view and click on them.
- Make sure the terms and conditions are enabled.
- Select "Applied" > "Manually".
The configuration "Applied" changes whether it's displayed as a checkbox or a link.
For more information on configuring terms and conditions in Magento 2, see the Magento 2 documentation.