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:

  1. Stores > Configuration - select correct store view.
  2. Sales > Checkout > Enable terms and conditions > "Yes"

Then, in order to render it as a checkbox and not a link:

  1. Stores > Terms and conditions
  2. Find the correct terms for the store view and click on them.
  3. Make sure the terms and conditions are enabled.
  4. 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.