I looked at a couple of articles/posts on Angular today. There is this great article on Stack Overflow:

Apparently this is for older versions of Opera as I had suspected. As of this year, versions of Opera are based on webkit, like chrome and safari. This is quite an interesting development in web design. It may even signal the end of vendor prefixes altogether. Here are some interesting articles I found on the […]

I went looking on the web for exactly what that -o- is for (older opera or opera mini) and found this great article: sitepoint.com: browser-trends-september-2014