I assume you mean IWD OPC? If not you can ignore this answer.
The checkout extension ships without translations. It should be fairly easy to generate those.
First, you need to create i18n directory inside the module so the structure is
<mage_dir>/app/code/Vendor/Opc/i18n
Then generate the dictionary with magento shell tool; from :
php bin/magento i18n:collect-phrases -o app/code/Vendor/Opc/i18n/en_US.csv app/code/Vendor/Opc/
Use the proper folder instead of 'Vendor'. Now you have a full dictionary for the module.
Finally, duplicate the en_US.csv
file in the i18n directory and rename it to es_MX.csv
.
Translate the content, clear caches and the phrases will be translated.