O

objectCenter
objectLayer 
objectType
objectTypeUI
objExists
obsoleteProc 
oceanNurbsPreviewPlane 
offsetCurve
offsetCurveOnSurface
offsetSurface
openGLExtension
openMayaPref
optionMenu
optionMenuGrp
optionVar
orbit
orbitCtx
orientConstraint
outlinerEditor
outlinerPanel
overrideModifier

indicates a MEL script