Create in Oxygen a new DITA topic in the Author visual editing mode. Open the HTML content in the web browser, select all, copy and then paste in the topic you created earlier. Oxygen will attempt to convert the HTML content to DITA.
This is also done with special XSLT code developed by Oxygen. The main XSLT for this is located in:
OXYGEN_INSTALL_DIR\frameworks\dita\resources\xhtml2ditaDriver.xsl
and you can also try to create a transformation for it and apply it to the XHTML content.
You can also convert HTML to XHTML by using Oxygen's File->Import->HTML File menu entry.