I've worked a fair bit with Oracle Spatial and I've had some requests to transition to PostgreSQL Spatial.
Is there a common interface for SDO_Geometry and ST_Geometry? I'm working in Java with EclipseLink ORM.
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GeoTools http://geotools.org provides PostGIS and OracleSpatial datastores which will abstract away the differences.
OGR supports both Oracle Spatial and PostGIS and there are Java bindings for OGR. Maybe OGR's ogr2ogr can help.