In some scenarios it is required to put a site in read only-mode, for example a project site, or to simply just remove access to it.
From the SharePoint Online Admin Center there is no way to lock your site, or to put it in read-only. There are two ways to achieve this, one is through Powershell, the other one through the Site Settings of your Site Collection.
A customer of mine had the need to get an overview of all the checked out documents for a given Site Collection, and to check them in afterwards prior to a content migration from SharePoint 2013 to SharePoint Online.
Since no out of the box features exist to accomplish this goal, I had to write a Powershell-script to find all the checked out documents in a Site Collection.
You can find the script here. It works for SharePoint Online as for SharePoint 2013. I did not test it with SharePoint 2010.