OIB Validator is a Chrome extension developed by Kristijan Modric. It is a free tool in the Browsers category, specifically under Add-ons & Tools. This extension is designed to check the validity of OIBs (personal identification numbers) used in Croatia. However, if you are outside of Croatia, this extension may not be useful to you.

To use OIB Validator, simply install the extension and right-click on any selected text that you would like to check for OIB validity. A menu item called "Check if OIB is OK..." will be displayed. Clicking on it will show you a message indicating whether the numbers are mathematically correct OIBs or not.

OIBs are created by generating 10 digits and then calculating the final 11th control digit, which is used to validate the OIB. The extension checks if the OIBs are mathematically correct, but it does not guarantee that they are actually used by citizens or companies in Croatia.

Privacy-wise, all calculations are performed locally on the client's device, and no personal information is shared over the internet.