SharePoint Online - Get Stats for User Actions\Visitor activities in Office 365

You can use the audit feature to track which users have taken what actions on the sites, content types, lists, libraries, list items, and library files of site collections. Moreover, you can use the audit log reports to view the data in the audit logs for a site collection.

Please refer to the steps as below to configure audit settings.
1.Click Site settings on the Site actions.
2.Click Go to top level site settings under Site Collection Administration, if you are not at the root of your site collection.
3.Click Site collection audit settings under Site Collection Administration.
4.Set Automatically trim the audit log for this site to Yes in the Audit Log Trimming section.
5. Optionally, specify a location to store audit reports before trimming the audit log.

Please refer to the following steps to view audit log reports.
1.Click Site settings on the Site actions.
2.Click Site collection audit settings under Site Collection Administration.
3.Select Audit log reports.
4.Select the report that you want, such as Deletion.
5.Type or Browse to the library where you want to save the report and click OK.
6.On the Operation Completed Successfully page, click Click here to view this report.

I would like to share some references about audit settings and view log reports with you.
http://office.microsoft.com/en-us/sharepoint-online-enterprise-help/configure-audit-settings-for-a-site-collection-HA102031737.aspx.
http://office.microsoft.com/en-us/sharepoint-online-enterprise-help/view-audit-log-reports-HA102039795.aspx?CTT=3.