The border-radius CSS3 property is used to create round corners on elements. Basically you could use it on most elements.

border-radius:10px;

On Opera it will work only if the element you apply it to has display:block, it won’t work on any inline elements.
So, let’s have this link:

<a href="#" class="mylink">Link</a>

Css definitions:

.mylink
{
     background: red;
     color: white;
     border-radius:10px;
     padding: 10px;
}

In Opera border-radius won’t work.
If you add display: inline-block; it will work in Opera too:

.mylink
{
     background: red;
     color: white;
     border-radius:10px;
     padding: 10px;
     display: inline-block;
}

You can add simply display:block, depending on what you need.