I assume you mean Oracle XE (Express Edition).

Yes, it is free for use subject to the license terms

http://www.oracle.com/technetwork/licenses/database-11g-express-license-459621.html

"We grant you a nonexclusive, nontransferable limited license to use the programs for: (a) purposes of developing, prototyping and running your applications for your own internal data processing operations; (b) you may also distribute the programs with your applications; (c) you may use the programs to provide third party demonstrations and training; and d) you may copy and distribute the programs to your licensees provided that each such licensee agrees to the terms of this Agreement. "

But one *critical* thing to be aware of here - XE is a "snapshot" of the software taken at a point in time. There are no patches, which means when *security* patches are released, you cannot apply them to XE. So for any *public* facing database, I think there is a risk associated with XE. For internal stuff, it is still a risk but more easily managed.