Cloned mouse embryo development to blastocyst stage correlates positively with the expression level of Oct4 (Pou5f1) at the morula stage, as reported previously by our laboratory. However, whether this correlation is based on a cause-effect relationship has remained unclear. To address this question, we artificially increased the level of Oct4 prior and subsequent to somatic cell nuclear transfer, by microinjection of Oct4 mRNA into ooplasts and by transgenic Oct4 induction at the morula stage, respectively. We observed higher developmental rates of cloned embryos to blastocyst when higher levels of Oct4 were superimposed with the initial reprogramming events; whereas increasing Oct4 at later stages of preimplantation development did not have a significant effect on developmental rates. Our results show that supplemental Oct4 facilitates oocyte-mediated reprogramming only during the first cleavages, implying that the higher Oct4 level observed in developmentally competent cloned morulae is a readout of reprogramming events that successfully took place earlier.