At the same time, Oracle is running the risk that some Siebel customers will turn to other vendors as it works on an integration plan.

'A number of customers are in a wait-and-see mode,' said Rob Bois, an analyst at Boston-based AMR Research Inc. 'They're evaluating potential alternatives' just in case Oracle moves in a direction they don't like.

On the other hand, said Bois, replacing Siebel wouldn't be an easy task because many users have heavily customized the software. Any migration to a new supplier could take years, he said.

'Obviously, there is risk [that users will flee] with any acquisition like this,' said Marc Hebert, executive vice president of Sierra Atlantic Inc. in Fremont, Calif., and a user of Siebel CRM OnDemand and Oracle Financials software.

Oracle must undertake the same type of outreach done with its earlier PeopleSoft acquisition to avoid losing customers, Hebert said. He said it's likely that customers are confused, noting that the combined company will have four separate CRM tools among other overlapping product lines.