Oxygen Theme Description http://theme.wordpress.com/themes/oxygen Live demo site http://oxygendemo.wordpress.com/
You do not post directly under either or those. The software automatically display your posts there. In Oxygen, there is no way to show posts in both the post slider (Showcase Page template) and the Recent Articles area. It's been designed to not to have duplicated posts on the page. Also note there is no option to display more than three Recent Articles on the Showcase Page template.
Please feel free to read the Oxygen theme description and the many threads on how to use the theme by clicking the links in the sidebar tags.
Oxygen Theme Description http://theme.wordpress.com/themes/oxygen/ Live demo site http://oxygendemo.wordpress.com/
In Oxygen you can either have a Home page that displays your posts or a static front page ie. a Showcase template.
Read > http://en.support.wordpress.com/pages/front-page/#setting-up-a-static-front-page
Showcase page template
When you first activate Oxygen, you’ll see your posts in a traditional blog format but if you’d like to showcase your content in an online magazine-like fashion, then you can use the Showcase Page template. ...
Sticky post thumbnails are 750 pixels wide in the featured slider
Regular post thumbnails are 450 pixels wide
"I can't find the 'Page Attributes' module for my 'Oxygen' theme"
To create a showcase template, you need to go to Pages > Add New. That's the dashboard screen that should be displaying the Page Attributes module. If it isn't, you click Screen Options http://en.support.wordpress.com/screen-options/ while on that screen (the page editor screen): and include all the Pages and Posts modules. http://en.support.wordpress.com/modules/#list-of-modules each dashboard screen has different screen options.