Essential biological parameters such as length-weight relationships, morphometric characterization of body parts, length frequency histogram and eye diameter, were investigated for three months, in ninety-eight (98) specimens of Oreochromis niloticus purchased from fishermen by the bank of Onah Lake, Asaba, Nigeria. Data analysis showed that the regression coefficients were significant for male (r2=0.634) and female (r2=0.704) specimens. Morphometric characterization of body parts showed that female specimens attained higher values for total length (14.68±.87cm), standard length (12.18±0.41cm), body weight (89.52 ± 6.28g), left eye diameter (0.63±0.02mm) and right eye diameter (0.62 ± 0.02mm) than their male counterparts whose values were 11.82 ± 0.56cm, 10.09±0.40cm, 64.71±6.69g, 0.53±0.02mm and 0.51±0.02mm for same afore-mentioned body parts respectively. The specimens attained one year old (1+) at a modal length of 71-81mm. Specimens that attained 2+, 3+, and 4+ years of age ranged between 93-125mm, 136-147mm and 148-220mm in body length respectively. Also, eye diameter increased by approximately 0.01mm with each unit increase in age, as shown by the values of 0.03, 0.05, 0.06 and 0.07 at ages 1+, 2+, 3+, and 4+ years respectively. It was concluded that although female Oreochromis niloticus had higher values in terms of morphometric characterization of body parts, regression coefficients involving these values were significant for both sexes. Moreover, eye diameter and standard length of the fish proved to be most conducive for determining the age of Oreochromis niloticus specimens from Onah Lake, Nigeria.