Salesforce is the master system for data. All the data is stored in Salesforce. An external system can consume oData.
Is it possible to use salesforce as the source of data and feed it to the external system?
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You could probably write Apex webservices that conform to the OData standard, but this may not be easily scalable as you'd need to spin up new classes for each type of object you add, unless you use the SObject name in the URL parameters and do everything dynamically.
If you do take this approach be sure to lock it down and make sure you're not opening your data up to the world!
Looks like you are trying to convert salesforce data into OData so that it can be consumed from another OData consumer. You could use an OData library like Olingo or you can try using a service like DataDirect Cloud . Try this Produce OData from Salesforce page.
Disclaimer: Please note that I work for Progress DataDirect.