The octet rule is insufficient for describing the covalent bonding of molecules. Using only the octet rule, the chemist might draw an oxygen or a nitrogen with four bonds around it. This would be incorrect. This is one of those instances.
Carbon prefers to have four bonds, and no lone pair of electrons.
Oxygen prefers to have two bonds and two lone pairs of electrons. Nitrogen prefers to form three bonds and have one lone pair of electrons. You will apply the concept of formal charge in Organic Chemistry when you begin drawing reaction mechanism. To see an animated reaction, try the Transesterificatoin in Polyester Synthesis page at the USM Polymer Science Department's Macrogalleria . Copyright 1998 by William L. Dechent.