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Right now, there is no way (as far as I know) to query for questions based on multiple tags using an OR constraint.
You can query for questions tagged with multiple of these tags, but not with one or many of them.
The tagged parameter to /search and related methods is an OR.
This query will return questions tagged c# or java, for example.
Or is rather expensive, which is one (of the many) reasons those methods are limited in how many results they return.