Hi,
I have done multiple integrations from OS with D365 as the source. I have used both the approaches you mention (OData & Synapse Link). However in the majority of cases OData has been the preferred approach. Typically the OData approach removes the requirement to use the Smart Integration Connector. For the data volumes typically being sourced OData performance is good and OneStream provides plenty of flexibility to handle larger data volumes when using a web service based integration pattern. We have also utilized D365 OData endpoints to synchronize metadata between D365 and OS using ACM if that is a requirement