The oxygen isotopic ratio (O-18) in tropical Pacific coral skeletons reflects past El Nino-Southern Oscillation (ENSO) variability, but the O-18-ENSO relationship is poorly quantified. Uncertainties arise when constructing O-18 data sets, combining records from different sites, and converting between O-18 and sea surface temperature (SST) and salinity (SSS). Here we use seasonally resolved O-18 from 1958 to 1985 at 15 tropical Pacific sites to estimate these errors and evaluate possible improvements. Observational uncertainties from Kiritimati, New Caledonia, and Rarotonga are 0.12-0.14, leading to errors of 8-25% on the typical O-18 variance. Multicoral syntheses using five to seven sites capture the principal components (PCs) well, but site selection dramatically influences ENSO spatial structure: Using sites in the eastern Pacific, western Pacific warm pool, and South Pacific Convergence Zone (SPCZ) captures eastern Pacific-type variability, while Central Pacific-type events are best observed by combining sites in the warm pool and SPCZ. The major obstacle to quantitative ENSO estimation is the O-18/climate conversion, demonstrated by the large errors on both O-18 variance and the amplitude of the first principal component resulting from the use of commonly employed bivariate formulae to relate SST and SSS to O-18. Errors likely arise from either the instrumental data used for pseudoproxy calibration or influences from other processes (O-18 advection/atmospheric fractionation, etc.). At some sites, modeling seasonal changes to these influences reduces conversion errors by up to 20%. This indicates that understanding of past ENSO dynamics using coral O-18 could be greatly advanced by improving O-18 forward models.