The contrast between Okonkwo's and Obierika's approach to culture can be paraphrased into two ideas. In Okonkwo's case, Culture DETERMINES his life and his choices. He does not try to maneuver any of the demands or orders from the Higher Powers in the tribe but instead simply follows them to the letter without complaint. In contrast, for Obierika, culture merely INFLUENCES his life choices as he openly questions some of the Ibo traditions and gives himself some room to maneuver around the instructions of the Oracle and other higher powers. Even though he does usually carry out the orders from the Ibo gods he does not allow himself to be restricted by their demands. In Obierika's case the rules of the clan affect his daily life to some extent but they do not necessarily shape and dictate every aspect of his life.
In conclusion, I believe that unlike Obierika, most of the Ibo tribe are like Okonkwo in the sense that they allow the tribe's culture to determine their decisions. It can especially be seen in the killing of Ikemefuna as well as the circumstances leading up to Okonkwo's exile, that culture has an integral role in the clansmen's decision making process within the novel. Therefore, in my opinion the Ibo culture and beliefs determine the everyday actions of the clansmen within the tribe and thus the Ibo culture influences the plot of "Things Fall Apart" to a great extent.