put the menu in tabs?
For this one, I would recommend searching for a tutorial until you find one you like and then adjusting the CSS selectors until you get the effect you want. I searched and found one possible example:
http://matthewjamestaylor.com/blog/beautiful-css-centered-menus-no-hacks-full-cross-browser-support
Their example menu uses an ID "#centeredmenu" for the base menu selector, but the Oxygen theme uses a class ".main-navigation" so you can just swap that out to make it work:
.main-navigation {
float:left;
width:100%;
background:#fff;
border-bottom:4px solid #000;
overflow:hidden;
position:relative;
}
.main-navigation ul {
clear:left;
float:left;
list-style:none;
margin:0;
padding:0;
position:relative;
left:50%;
text-align:center;
}
.main-navigation ul li {
display:block;
float:left;
list-style:none;
margin:0;
padding:0;
position:relative;
right:50%;
}
.main-navigation ul li a {
display:block;
margin:0 0 0 1px;
padding:3px 10px;
background:#ddd;
color:#000;
text-decoration:none;
line-height:1.3em;
}
.main-navigation ul li a:hover {
background:#369;
color:#fff;
}
.main-navigation ul li a.active,
.main-navigation ul li a.active:hover {
color:#fff;
background:#000;
font-weight:bold;
}
You can also adjust all of the colors and spacing values in the example if you'd like.
http://0to255.com/