Abstract: A substructure for an oil derrick comprising a base frame, a top frame and an intermediate bifurcated support frame consisting of a pair of horizontally extending traveling frames. A linkage network of swingable leg members intercouples the base, traveling and top frames for movement in parallelism between collapsed and elevated conditions of the substructure. In the collapsed mode the frame members are positioned in an adjacent relationship for presenting a low profile to a flatbed truck which allows a portable oil rig to be easily shifted onto the top frame and into alignment with the oil wellhead therebelow. A block and tackle assembly operably engages the traveling frames and is coupled to the derrick hook carried by the traveling block of the oil derrick. Upon operation of the latter the swingably mounted traveling frames move towards each other causing an elevation of the top frame so as to present a working space between the elevated oil derrick and oil wellhead.