We have not tried that, but I read a discussion on Metalink, that although some components may work, it is not a certified combination.
Here is an excerpt of the discussion:
“We manage a 10g database using 9i OEM and console. You just need to do a minimum install of the 9i components on the server housing the 10g database (SQL*Net, Intelligent Agent, SQL*Plus, etc.), then start up the 9i Agent.
Copy the listener.ora from the 10g/network/admin directory to the 9i/network/admin directory, so the agent can discover the database services running on the node.
Granted you will not be able to do everything through the 9i console with the 10g database, but it is possible to do most things, including alerts, scheduling jobs, browsing the schema, etc. “