The Opera Browser Prefix: -o-
The same as the spec.
the -o- prefix used by opera uses exactly the same syntax as the unprefixed commands and it's not clear when you would ever actually need to use the Opera specific prefix.

However, Opera recognizes:

    linear-gradient
    -o-linear-gradient
    -webkit-linear-gradient

    repeating-linear-gradient
    -o-repeating-linear-gradient
    -webkit-repeating-linear-gradient

but not:

    -moz-linear-gradient
    -moz-repeating-linear-gradient
Which is weird: "all of them except -moz". Roll eyes, facepalm.

Opera recognizes only unprefixed radial commands: radial-gradient and repeating-radial-gradient until adoption of the Chromium rendering engine and now accepts WebKit versions of radial gradient commands, but older Presto versions of Opera do not support WebKit radial commands, only some the linear ones - the WebKit versions of repeating linear gradients are not recognized, but the non-repeating WebKit linear gradient commands are recognized.

Be aware though, if you are using Opera and you have conflicting commands, it will use the most recent one it successfully parsed, not the most correctly named one. A good example is to find some piece of code that has all the browser prefixes, then change only the Opera one and watch how Opera renders it. Unless it's the last one parsed, it'll pick up some other value.

That is:

    radial-gradient (circle...
    -o-radial-gradient (circle...
    -webkit-radial-gradient (ellipse...
    -moz-radial-gradient (circle...
...will render an ellipse in Opera, not a circle as you might expect. This is a nasty side effect of multiple browser prefixes when browsers recognize some or all of them. Opera recognizes all of these except Mozilla, and an unprefixed form and the -o- form don't have preference over the WebKit form. It can be a bit a messy if you aren't careful.

All of the Opera prefixed and unprefixed commands have identical syntax and semantics for both commands and argumentsand the gradient commands are exactly identical to the unprefixed ones; the Opera browser prefix is the only one that is identical to the unprefixed commands implemented per the W3C spec.