OpenACS 5.3.0's search initialization script always scheduled the Oracle indexer regardless of database, which was a bug.
OpenACS 5.3.2 (which has just been built) fixes that bug.
If you don't want to update, just go to
search/tcl/search-init.tcl
and delete the call which schedules the tcl proc which calls the Oracle Ctx* proc, and restart your server.