How do i extract (reverse engineering) a graphical (schema) representation of the Oracle database (tables as well as their associations with colums and datatypes....). Exist (free) tools, which could do that?

I believe the Oracle SQL Developer doesn't have such feature...

There's an associated Oracle tool, SQL Developer Data Modeler which does reverse engineering which is free. Find out more.

Sql Developer performs this (look into the SQL Modeller option) and plenty much more. An incredibly good tool !

For any free tool I suggest schema spy, http://schemaspy.sourceforge.net/. It requires Java Runtime. Turn to http://schemaspy.sourceforge.net/sample/relationships.html to determine diagrams it generate.

It can also be more appropriate to automatic build scripts.

simple usage with oracle hr template is offered below.

SET JDBC_ORACLE_JAR=C:\oracle\product\10.2.0\client_1\jdbc\lib\ojdbc14.jar
SET CONNECTION_USERNAME=hr
SET CONNECTION_PASSWORD=hr
SET TNS_DATABASE_NAME=orcl
SET SCHEMA_NAME=HR
SET TABLE_NAME='EMP'
SET OUTPUT_NAME=HR_EMP

java -jar schemaSpy_5.0.0.jar -dp %JDBC_ORACLE_JAR% -db %TNS_DATABASE_NAME% -o     schemaOutput%OUTPUT_NAME% -u %CONNECTION_USERNAME% -p %CONNECTION_PASSWORD% -i %TABLE_NAME%.* -schemas %SCHEMA_NAME% -hq -noviews -loglevel severe

java -jar schemaSpy_5.0.0.jar -dp %JDBC_ORACLE_JAR% -db %TNS_DATABASE_NAME% -o     schemaOutput%OUTPUT_NAME% -u %CONNECTION_USERNAME% -p %CONNECTION_PASSWORD%  -schemas %SCHEMA_NAME% -hq -noviews -loglevel severe

First one provides you with diagram with tables beginning with EMP . Second one provides you with diagram with all of tables in hr schema .

Adminer is definitely an awesome PHP single file which should have the ability to do that which you request. I attempted just with MySQL, but Oracle it's supported too.

I am unaware of any free tools: MS Visio can reverse engineer (professional/Premium) TOAD an costly tool but worthwhile IMO