North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are two large-scale atmospheric oscillations in northern and southern hemisphere respectively. These two oscillations are known to affect Indian summer monsoon (June-September) rainfall (ISMR). It is logical to expect the combined effect of these two oscillations on monsoon rainfall over Indian subcontinent. In the present paper this combined effect of NAO and SO on Indian summer monsoon rainfall is studied. North Atlantic Oscillation (NAO) index in the month of April is showing an inverse and statistically significant relationship with Indian summer monsoon rainfall (ISMR). Southern oscillation (SO) indices from April through December show direct association with ISMR. The present study deals with the simultaneous effect of NAO and SO on monsoon rainfall over India. The effective strength index is newly defined to quantify the simultaneous effect of NAO and SO and its relationship with ISMR is investigated. For this purpose 40 years data (1951--90) has been used. The study emphasizes an important role of interaction between NAO and SO in the month of April. It also emphasizes that monsoon activity over India depends upon the strength of both the oscillations namely NAO and SO. The analysis suggests that the effective strength index can be used as a pre-cursor to understand the extreme monsoon events. The analysis also suggests an inverse association of ISMR with the effective strength index from April through December with statistically significant relationship in the months of April, October, November and December. ¿ 2000 American Geophysical Union