object
ordered pair
ordered triple
obsolete
operation
of
Some six letter words starting with O are:objectoblongoccultocelotoddityoffendofficeorangeorioleorphanoverdooxygen
A few examples of 3-letter words starting with the letter O are:owloweoaroatown
or, out,
ortaments
our outerwear
The word older - in mathematical words or otherwise - means starting from an earlier time.
slow
obsessoppressedopposeoverratedottomanoutclassoutriggeroverreactofficiousoverranoverreachoffsetoffshoreofflineomissionoverrideoppositeoffbeatoddoddityoutselloverallsopportunityossifyothernessotterodorlessoddmentsoffaloffendoffenseofficeoffhandofficialofficerofficiate
overpowering
Overflowing, ominous, erupting.
Outstanding is a nice word. It begins with the letter O.
A few words starting and ending with a consonant that have o in the middle are:bogboxboycodcogcotcoycowdogdotfogfoxfobgobgodgothothobhophowjobjotloblotloglowmowmobmodmommopnotnowpowpodpoppotrotrobrodrowsobsodsonsowsoytogtomtontoptottowvowwow