Here is an example you can start with that will adjust the widths of the main content area and the left and right sidebars in the Oxygen theme:
#page {
max-width: 1040px;
}
#content {
margin: 0 21% 0 24%;
}
#secondary {
width: 23%;
}
#tertiary {
margin: 0 0 0 -20%;
width: 18%;
}
Note that since this approach updates the overall width, you won't be able to use Appearance → Header page to set a full-width header image any more. You can add one manually though, here's how:
#masthead a img {
display: none;
}
#masthead > a {
background: url('http://providencecommunityacupuncture.files.wordpress.com/2013/02/newnewnewnewnew.jpg') center;
display: block;
width: 1040px;
height: 294px;
}
Note that the width value should match the #page width from the example above. Also, the image repeats, but it looks pretty good with your current image. If you didn't want the image to repeat, you could add "no-repeat" just before ";" in the background property line or you could use an image that is at least 1040px (or whatever width you set) for the url() value.