We did one many years ago using Boomi. It was complicated and often had issues (not necessarily due to Boomi though, just due to general complexity). I would recommend doing a one-directional integration, if possible. If you need bidirectional, for each field, choose which system will win instead of trying to handle conflicts with complicated rules. Finally, if you can automatically import/export CSV files instead of using APIs, that would simplify things too. Marketo Consulting generally wouldn't take on a completely custom Marketo-Oracle integration, but we do offer a CSV import/export service. Obviously, it is possible to do a fancy and full-featured integration, but it will take a lot of development time and someone who is familiar with the Siebel or Oracle on Demand APIs and architecture (and Marketo).
Thank you for your feedback, In this integration I will need to perform bidirectional integration. I will definitely take your feedback on board