Hey guys, I had exactly the same problem, but found a first step to solve it:
The search relys on Oracle Text. This is why the wiki db user needs to have CTXAPP rights (can also be seen in file maintenance/oracle/user.sql):
SQL> grant CTXAPP to ...;
I ran into this problem, because I didn't use the wizard to create my user (which didn't work and I was too lazy to step into the reasons), but created it myself. After granting the above role, I had to manually rebuild the indexes - I hope this will work from scratch, if you add the role at the beginning.
Unfortunately I'm still unsatisfied, because the search only finds terms in lower case. Meaning you have to enter the search term in lower case, the results are case insensitive then...
Again to workaround this, I modified the line containg CONTAINS in includes/search/SearchOracle.php to surround $$searchon with LOWER:
return " CONTAINS($field, LOWER($searchon), 1) > 0 ";
Now it seems to find any search string, but I'll have to keep an eye on index rebuilding...