Exceptions are supported but must be declared in the input file (so that they can be marshalled). Unfortunately it seems not to be possible to equate one exception with another and also define the data it contains, so built-in exceptions cannot be used. (XXX: this is not true, it is possible, but it is not yet supported in orpc.)
When any exceptions are declared in the input, then the calling convention for the entire interface is changed: each return value is wrapped in an 'a orpc_result, defined as Orpc_success of 'a | Orpc_failure of exn, to accomodate the possibility of exception. This doesn’t affect the external interface, but means the wire protocol is not compatible.
An exception raised on the server which is one of those declared in the input is marshalled and raised on the client. Others are reported as a generic transport-level exception in the usual way.