Changing any text on the screen is like any regular Magento translation, you need to change or add the string in locale files, /www/app/locale/your locale/Idev_OneStepCheckout.csv
Please note that if the string that you want to translate does not exist in your locale file yet, you need to add it in the same format as you see the existing translations in the file.
In case translation does not work, go through this checklist
(1) Have you flushed Magento cache and any other caches that you may be using, like OPCache, XCache, FPC, Varnish, etc
(2) If still not working, use text search to search all locale files if same string is translated twice and if there are duplicates that override your translation
(3) In some cases strings may be hardcoded into template files, check template file. This is not the case with clean One Step Checkout package but if your One Step Checkout template files are customised, it may make sense to check this.