If you are using Chrome, you may not be able to preview .doc files because Chrome uses HTML5. You can convert these to .docx files to use the preview function as well as to edit it using Office Online. To convert a file, click 'save as' and select .docx in your desktop Office software.
If you are using Firefox, you should be able to preview .doc files and .docx files because Firefox uses Java.
*Note: Make sure that your collaborators are also saving as .docx when collaborating on documents.