Migrate Sybase table definitions, indexes, primary and foriegn keys, constraints, triggers, functions, views, procedures and data to Oracle using SQL Developer
http://www.oracle.com/technology/tech/migration/workbench/viewlets/sqlserver.html
The above uses JDBC to pump the data from Sybase to Oracle.
This has its limitations, so why not use SQL Developers automatically generated BCP and SQL*Loader scripts to pump data from Sybase to Oracle
http://www.oracle.com/technology/tech/migration/workbench/viewlets/ofdm.html
Still , you would have to take your database down for some time while the data move was acomplised. Thats where Oracle GoldenGate comes in. It allows real time data move without taking down your Sybase database , and ... even when your apps running on Oracle you can keep the data updated on your Sybase system until your happy to turn it off.
http://www.goldengate.com/solutions/migrations.html
Sorry youll have to read that page. Ill put together a viewlet soon.
GoldenGate also offer an impressive data comparison tool.
SQL Developer is free to download off OTN
http://www.oracle.com/technology/products/database/sql_developer/index.html
Oracle Golden Gate is not free
http://www.oracle.com/goldengate/index.html