If a NAND gate is universal, why you don't have NAND OISCs If a NAND gate can be used to construct all other of the basic logic gates, then I'm wondering why you don't/can't have a purely NAND-based One Instruction Set Computer (OISC). All the OISC single instructions are complicated like "addleq, add and branch if less than or equal to zero", but I don't see why you can't just have an OISC be "nand, perform nand operation". Wondering what I am missing.
With such an instruction set, all you could express are straight-line programs.  Without branches, you can't have loops.  Thus, the program would not be able to handle arbitrary-length inputs: it would be limited to dealing with fixed-size inputs (or inputs with a fixed known upper bound on the size of the input).  So, that wouldn't be very satisfactory.