Why is my Table of Contents created empty in OpenOffice/LibreOffice?
According to OpenOffice wiki every item which you want to see in the Table of Content must be styled as heading. So change style of titles of paragraphs to heading (1,2,3 etc.) and you will see them in the Table of Content.
Then right-click on the Table of Content and select Update Index/Table from the pop-up menu.
Seems there is a bug on Google Docs. All the downloaded ODTs lack of the relation between the outline and the headings styles. Here is a similar question. Therefore, you have to fix the issue:
Go to Tools > Chapter Numbering... (Outline Numbering on older versions), and select the paragraph style (probably "Heading 1" for Level 1 and so on... up to level 6.
Then update the ToC:
Update: It seems fixed on newer versions (6.0+).