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OISC

One Instruction Set Computer.  A parody of RISC (Reduced Instruction Set Computer), in which there is only one instruction: Subtract and branch unless positive.

Oroogu by Georg Kraml

A language developed on the model of Q-BAL, with some added queue manipulation operations and a completely redesigned control flow mechanism.

Orthagonal

The predecessor to Orthogonal, which differs by a bug in the specification.

Orthogonal by Brian Raiter

A two-dimensional fungeoid developed independently of and around the same time as Befunge-93.  Unlike Befunge, instructions are not necessarily single characters, but written in 'cells' a few characters wide.