We consider the problem of evaluating the cumulative distribution function (CDF) of the sum of order statistics, which serves to compute outage probability (OP) values at the output of generalized selection combining receivers. Generally, closed-form expressions of the CDF of the sum of order statistics are unavailable for many practical distributions. Moreover, the naive Monte Carlo (MC) method requires a substantial computational effort when the probability of interest is sufficiently small. In the region of small OP values, we propose instead two effective variance reduction techniques that yield a reliable estimate of the CDF with small computing cost. The first estimator, which can be viewed as an importance sampling estimator, has bounded relative error under a certain assumption that is shown to hold for most of the challenging distributions. An improvement of this estimator is then proposed for the Pareto and the Weibull cases. The second is a conditional MC estimator that achieves the bounded relative error property for the Generalized Gamma case and the logarithmic efficiency in the Log-normal case. Finally, the efficiency of these estimators is compared via various numerical experiments.[13] distributions. On the other hand, efficient simulation methods have been also developed for the estimation of the CDF of the sum of RVs such as the Log-normal [3], [14]-[17] and the Generalied Gamma [3].In the general case where L < N and apart from the exponential and Gamma RVs, closed-form expressions of the CDF of partial sums of ordered RVs are out of reach for many challenging distributions and are still open problems. This is for instance the case of the Log-normal RV which models shadowing [18] and weak-to-moderate turbulence channels in free space optical communication systems [19]. The Weibull variate, which has also received an increasing interest and has been shown to fit realistic propagation channels [20], is another example where the CDF of sums of order statistics is not known to possess a closed-form expression. Thus, it is important to propose alternative approaches to compute the CDF of sums of ordered RVs with arbitrary distributions.