The basic difference is that in teradata we have the concept of AMPs which takes care of the part of database virtually assigned to it. It takes care of all the operation relating to the data which is in that AMP. This is not the case of oracle
Even though oracle supports SMP and MPP it doesn't have shared nothing architecture but a distributed architecture.
In Oracle Hexadata there is parallelism in the top of the architecture, but there is a bottleneck behind it and it's not parallelism from this part to the core. Pls, have a look in the Oracle Hexadata Architecture and you can see it.