The purpose of the present study was to investigate (a) the extent of Omani children acquisition of some cognitive operations (conservation of number, conservation of length, conservation of quantity, seriation and class-inclusion) in concrete operational stage, (b) the most prevalent explanations which Omani children gave when they answered correctly conservation tasks (number, length and quantity), and (c) the effects of gender, age and the interaction between these two variables, on the development of cognitive operations in the Omani environment. The instrument of the study was administered which consisted of Piagetian tasks of the five cognitive operations, to a sample of (180) children (balanced for sex) from the ages of 6 to 12 years, who were taken from basic education schools in Oman. Results indicated that: (a) Omani children acquired cognitive operations (conservation of number, conservation of length, conservation of quantity, seriation and class-inclusion) in ages (8.5-9.5, 8.5-9.5, 10.511.5, 8.5. 9.5, 10.5- 11.5) respectively, (b) the most popular explanation which children gave to their correct answers of conservation tasks was "identity", and the least popular explanation was "compensation" , and (c) there were statistically significant differences (a = 0.001) in the five cognitive operations among children which were attributed to their ages, but there no statistically significant differences in the five cognitive operations among children which were attributed to their gender or to the interaction between gender and age.