Update 21st March 2019
You can now find these settings in the OneDrive Admin Center (Preview) at https://admin.onedrive.com and that’s a clearer experience.
Every organisation has their own requirements and standards. For mine, I see a risk when the default action of sharing a document via OneDrive for Business is the ability to ‘Allow editing’ of any document sent out. It’s worse because that option is hidden behind the main popup when sharing a file, and you don’t actually see that you’re giving ‘modify’ access rather than ‘read only’:
There is a way to change this default behavior though, and it’s not in the OneDrive admin center.
Instead, you’ll need to head to the SharePoint admin center (since the backend of OneDrive is SharePoint Online, this makes some sense). From here, go into ‘sharing’ and there’s an option around ‘Default link permissions’. You can change this to ‘View’ rather than ‘Edit’:
The change was immediate from my testing, as soon as I went to share another file via OneDrive for Business, the ‘Allow editing’ option was unticked. This is only changing the default too, someone can still decide they want to allow editing and tick the box.
It’s worth considering what you should have as your default. The new versioning in OneDrive/SharePoint Online is really good, and will let a user easily roll back to a previous version of a document if something accidentally gets changed – but will your users be aware if something does change? It’s possible to set up an alert, but it’s a bit tedious: http://itgroove.net/brainlitter/2016/05/16/creating-alerts-documents-new-onedrive-business/
Hope this helps anyone considering rolling out OneDrive, or wants to start allowing external sharing.