I was searching for OpenSolaris but landed in Oracle's website of Solaris 11 Express. Is OpenSolaris replaced by Solaris 11 Express?
You are correct; Oracle is no longer contributing to OpenSolaris (reference). Oracle Solaris 11 Express releases more often than the standard build of Solaris and includes newer technologies. It may be used free of charge for development and testing, but not for production. Production use requires purchasing some form of Oracle Premier Support (reference).
OpenIndiana is an open-source continuation of the OpenSolaris project by the Illumos Foundation.