Sometimes electrical submersible pumps (ESPs) are deployed in oil wells to compensate for low reservoir pressures. Although such an artificial-lift mechanism dramatically boosts production rates, there are consequences on the quality of transient-pressure data in some cases. The paper presents the results of a thorough investigation of the behavior of pressure-transient data from horizontal wells equipped with ESPs in an oil field. Capturing the transient data without any distortion is important in estimating parameters of a horizontal well. This ensures establishment of all the important flow regimes on the log-log plot. But the early-time data suffers from distortion in buildup tests due to a very short period of hydrostatic balancing of fluids in the production string right after the ESPs are turned off. Drawdown data and rate transient analysis tools have been utilized to retrieve key well and reservoir information which might be missed in the early-time buildup data. Such integration is enabled by large sets of data from permanent downhole monitoring systems. Distortion of the early-time buildup data due to the falling liquid levels and some natural flow right after the ESPs are turned off for buildup tests consequently masks the early-time, vertical radial-flow regime. This distortion sometimes makes it impossible for unique estimates of the effective horizontal well length, the anisotropy ratio (vertical to horizontal permeability) and the damage skin. However, the empirical investigation shows that the early, vertical radial-flow regime develops in some drawdown tests with minor ESP effects. This drawdown data has been utilized to offset or reconstruct the flow regimes missed by the distorted early-time buildup data. In some cases, any low sampling frequency, low gauge resolution or lack of repeatability of the measured intake pressures at the ESP may hinder development of the early, vertical radial-flow regime in some drawdown tests. Note that a high sampling frequency and a high gauge resolution is required in capturing this flow regime. We have utilized rate-transient analyses to complement the pressure-transient analyses results whenever large data sets are available. The literature is devoid of any discussion on the challenges of interpreting the buildup tests on oil wells equipped with ESPs. This investigative paper with examples makes a comprehensive effort to understand such challenges. We have also shown ways to get around the problems and have provided reasonable interpretation of transient-test data.