Correct, the Opportunity API is only exposed for instances that do not have a native CRM sync enabled. What is your use case for needing to call the Opportunity through Marketo instead of SFDC API?
Most use cases where the Marketo user needs access to the Opportunity can be solved with velocity scripting which pulls back any visible fields and can cycle through Opportunities to render content in an email. Otherwise I suggest hitting Salesforce's API directly and join on a Marketo key.
Thanks, Wyatt. Sounds like I'll go with the plan you outlined in joining the two sources -- my goal was to pull all of the interesting moments for contacts on a particular SFDC opportunity and bridge it with the opp's data. I have to pull data from SFDC either way, I just wanted to learn more about the limits from the Marketo side.