Hydraulic fracturing is the most prominent technique for increasing well productivity in shale oil and gas reservoirs. Spacing between perforation clusters, with Plug-and-Perf (PnP) fracturing method also believed to be spacing of fractures, is one of the parameters that need to be optimized in fracturing design. This work presents an analytical method to optimize fracture spacing based on the assessment of production data from multi-fractured horizontal wells. Five hydraulically fractured horizontal wells completed in a high clay-content shale formation were considered as examples in this study. Based on the theory of pseudo-steady state (PSS) flow, oil well productivity was investigated with special focus on perforation cluster spacing. Wells exhibiting linear flow in rate transient analyses are believed to have a potential of productivity improvement with shorter fracture spacings. Oil productivity potential with closer fracture spacing was identified for wells in the same reservoir. Result of analyses shows that some wells experienced linear transient flow, indicating significant separation between hydraulic fractures. Use of an analytical well productivity model to simulate pseudo-steady production indicates that fracture spacing could have been reduced to improve well productivity. These wells can be considered as candidates for re-fracturing if possible. Future wells to be drilled in the area nearby should be completed with shorter spacing of perforation clusters. Some wells in the case fields do not show linear transient flow, indicating interferences between either hydraulic fractures and/or natural fractures. These wells should not be refractured. New wells near these wells should not be completed with fracture spacing less than the spacing values used in the existing wells.