Ethiopia has over one million hectares of highland and lowland bamboo resources. However, its utilization is fundamentally rudimentary, and its socio-economic and ecological potentials are not yet realized. In addition, bamboo has a great potential for climate change mitigation and adaptation. Hence, the objective of this study was to develop allometric models for estimating above and belowground biomass of lowland bamboo (Oxytenanthera abyssinica (A. Rich.) Munro. in Dicho forest. Thirty four individual bamboo plants were randomly selected from 3 plots of 100m2 for biomass data collection. Selected plants were extracted out; their basal diameter, DBH and total height were measured using tree caliper and diameter tape after felling the bamboos. The plants were then sorted into four components as rhizome, culm, branch and leaf. Total fresh weight of each part was measured immediately. Subsamples of 15-50 g for leaf and branch, and 100-300 g for stem and rhizome were then taken to the laboratory of Wollega University for dry to fresh weight ratio determination. The subsamples from culm, branch and rhizome were then dried at 105oC and those from the leaf were dried at 70oC in an oven until constant weight was reached. Total dry weight was determined using the dry/fresh weight ratios of subsamples. Basic wood densities of all culm and rhizome discs were determined. The wood volume was determined using the water displacement method. Wood density was then calculated from the ratio of sample dry weight to sample fresh volume. The harvested bamboos were classified into different DBH classes and their biomass share was computed. Linear regression techniques were used to develop allometric models from DBH, height, basal diameter, BA, wood density and their interactions to predict the total above and belowground biomass of bamboo plants. The best models were selected based on residual standard error and the value of coefficient of determination. Results show that the ratio of dry/fresh mass is highest for culms, with and the lowest is for foliage. The mean density of culm is calculated to be 0.49g/cm3 and that of the rhizome is 0.37g/cm3 . The underground biomass contributed 20.08% of the total biomass. The ratio of belowground to above ground biomass of O.abyssinica is 1:4 (25%). Moreover, the mean dry biomass (in Kg/plant) is calculated to be 2.17, 0.35 and 0.3 Kg for culm, branch and leaf respectively. Regression models indicated that DBH and D10 were found to be the best predictors for TAGB and TBGB respectively.