Blocking microarchitectural (digital) side channels is one of the most pressing challenges in hardware security today. Recently, there has been a surge of effort that attempts to block these leakages by writing programs data obliviously. In this model, programs are written to avoid placing sensitive data-dependent pressure on shared resources. Despite recent efforts, however, running data oblivious programs on modern machines today is insecure and low performance. First, writing programs obliviously assumes certain instructions in today's ISAs will not leak privacy, whereas today's ISAs and hardware provide no such guarantees. Second, writing programs to avoid data-dependent behavior is inherently high performance overhead. This paper tackles both the security and performance aspects of this problem by proposing a Data Oblivious ISA extension (OISA). On the security side, we present ISA design principles to block microarchitectural side channels, and embody these ideas in a concrete ISA capable of safely executing existing data oblivious programs. On the performance side, we design the OISA with support for efficient memory oblivious computation, and with safety features that allow modern hardware optimizations, e.g., out-of-order speculative execution, to remain enabled in the common case. We provide a complete hardware prototype of our ideas, built on top of the RISC-V out-of-order, speculative BOOM processor, and prove that the OISA can provide the advertised security through a formal analysis of an abstract BOOM-style machine. We evaluate area overhead of hardware mechanisms needed to support our prototype, and provide performance experiments showing how the OISA speeds up a variety of existing data oblivious codes (including "constant time" cryptography and memory oblivious data structures), in addition to improving their security and portability.