Open reading frame (ORF): Any stretch of DNA that potentially encodes a protein. Open reading frames start with a start codon, and end with a termination codon. No termination codons may be present internally. The identification of an ORF is the first indication that a segment of DNA may be part of a functional gene.

Object-oriented programming (OOP): Computer languages and programming philosophy that emphasizes modularity among the elements of a program and their sharing of properties and intercommunication.

Open Database Connectivity (ODBC): A framework for accessing and altering the contents of databases, developed by Microsoft Corporation.

Organism: An individual, composed of organ systems (if multicellular). Multiple organisms make up a population.

Orthologous: Homologous sequences in different species that arose from a common ancestral gene during speciation.

Overlap: A part of a sequence that is common to sequences derived from separate sequencing experiments.

Overlapping genes: Two genes whose coding regions overlap.