Migration Tools

Grab a cup of tea and kick back and prepare to "research" migration technologies without reading 1 doc or white paper. ehhh , ok theres one doc, or you could wait a few weeks till I get a viewlet together

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