ABSTRACT ODDA is an expert system designed to assist the drilling engineer, at the drilling site, in the engineering base or in the research center during the 4 functional phases of his work. Early planning phase, for designing the number and position of the different platforms or well clusters. (Field planning expert system). During a well planning phase, for determining the optimum well trajectory in a single or multiwell environment. (Trajectory planning expert system). During the operational phase: 3-1) for designing the bottom hole assembly suited to drill a given well section while minimizing the BHA make up time and implying no restriction on the drilling parameters to be applied. 3-2) for deciding to pursue or interrupt the current bottom hole assembly run, ODDA will evaluate and extrapolate the assembly's trends, account for operational and geometrical constraints and eliminate unnecessary trips. (Drill Ahead/Pull Out Of Hole expert system). During the evaluation phase, for running queries and statistics, comparing actual and predicted behaviours and identifying trends in the effects of different formations and drilling parameters on bottom hole assemblies (Post Analysis Module). The post analysis facility includes a survey validation process consisting in visually aided or fully automatic filtration of survey errors. The drilling engineer can also design his own assemblies by accessing the mechanical analysis module. This module is an upgraded version or TOTAL’S BHA2 program, which includes an enhanced graphical interface, a link with the system's central database and the possibility to run batch calculations while working with other functionalities. The system's relational database allows the recording and processing of the well programs, the prognosed and actual geological data, the drilling parameters, the surveys and associated error parameters, the bits and bottom hole assemblies. Reports of predefined or customized format can be generated at any time. Its graphics capabilities and user environment, together with the above functionalities, make the ODDA system an integrated tool covering most aspects of directional drilling engineering.