How do I track down wait events for a specific transaction in Oracle Enterprise Manager?
The ASH report shows top wait events, top SQL and top sessions. Using Enterprise Manager, you can generate the ASH report by navigating to the performance page and then clicking 'Run ASH Report'. On thhe performance page, you can also click the 'Top Activity' link. On the Top Activity page, you can then choose a time interval by dragging the shaded area(may have different drag controller version to version and browser to browser), and then click Run ASH Report.
For details:
Identifying High-Load SQL Statements