On an incline
On an ocean journey
On an ocean liner
On an ocean liner, e.g.
On an ocean liner, maybe
On an ocean liner, perhaps
On an ocean liner, say
On an ocean trip
On an ocean voyage
On an opponent's field
On an upper floor
On an upper storey
On an upswing
On and after
On and on
On and on ... or how to read 18-, 27-, 37- and 51-Across to understand this puzzle's theme?
On and on and on
On and on and on and on and ...
On and on and on and on and 
on and on Going Coming or
On another call
On antonym
on any occasion
on approval
on april 19 1775 hundreds of british
on assignment
on bad terms
On balance
On base
On base, in baseball
On base, say
On bed rest, say
ON behalf of
on behalf of king
ON BENDED
On bended ___
On better behavior, to Santa
On Beyond Zebra! boy Conrad Cornelius o'Donald ___
ON board
On board a ship or boat
on board Drink
on board Put
on board Room
on board Safe deposit box item
ON board ship
on board Stored
on bodybuilders Ripple
On both banks of
On both feet
On both sides
On both sides of
On break, say
on Bring about
on Build
on Call
On call device
on cargo Put
on Carry
On chairs
On Chesil Beach novelist Ian
on Chinese menus General address
On clippers?
On close personal terms
On cloud __
On cloud ___ (elated)
On cloud nine
On cloud nine feeling
On cloud Nine Group of
on Coat
On condition
On Contradiction author
On court stabbing victim
On deck
on deck Longer
On deck, for example
On deck, on ships
On deck, perhaps
On deck, say
On deep water
On delay
On delivery?
On different sides
On disk
On Disk Clear, as a
On disk, say
On display
On dope
on draft Drink
On drugs
On drugs, say
On drugs, slangily
On dry ground
On dry Land
On dry land
on duty
on each occasion
on earth
On Earth ___ is in Heaven
on earth Anywhere Not going
on earth Heaven
On easy street
On easy street, e.g.
On eBay, e.g.
On edge
On edge with excitement
On edge, as nerves
On edge, with "up"
On edibles, say
On either side of the center
On empty
on end
on end Time
on equal
On equal terms
on equal terms by payment or requital
On even terms
On every occasion
On every single occasion
On everyone's A-list
on face paint
On familiar ground
On fast food, perhaps
on fire
On fire in real time?
On fire in the kitchen
on fire Pants
On fire, as a match
On fire, as some desserts
On fire, in restaurant lingo
On fire, on a menu
On fire, so to speak
On firm ground
On first impression
On First Looking Into Chapman's Homer poet
on first or second or third base
On floppy disk, say
on focus
on foot
on foot Measure
On Friday, Big Apple ball park makes a bundle (5)
on friendly terms
On Golden ___
On Golden ___ (Henry Fonda film)
On Golden Pond actor or actress
On Golden Pond bird
On Golden Pond birds
On Golden Pond Oscar winner
On Golden Pond playwright Thompson
On Golden Pond role
On Golden Pond wife
on good
On good grounds
on good terms
on Grow dark
On guard
On guard against
On guard and alert
on Hancock, John
On hand
on heat
On her own
On hiatus
On High
on high
ON high (Gk.)
on high Home
On hold
On hold, with "in"
On holiday
On holiday, say
On horseback
on horseback Game 32-card
On ice, maybe
On impulse
On in years
On intimate social terms with
On intimate terms
On Irish author said to be merry
on it Count
on it Plan
on it you're dry
On it, captain!
On it!
On its last legs
On its way
On its way (or so they say)
On its way to the body shop
On its way, after "in"
On its way, as a fax
On its way, as a letter
On its way, as a message
On its way, as an email
On its way, in a way
On key
On Land
On land
On land (not at sea)
On land, as a 44 Down
On Language columnist
On leave
On leave, for a swabbie
On leave, maybe
On liberty
On liberty, e.g.
On like a lantern
on line maybe
On line shop keeper
on many occasions
On Medicare, say: Abbr.
On menus, it's often by the lobster
On mom's side
on Move
On my ___ ... (Girl Scout promise)
On My ____ (We Five hit by our Sylvia Tyson)
On my honor!
On My Own singer Patti
On Narcissism author
On naval maneuvers
On naval maneuvers, in a way
On naval maneuvers, say
on neck
On neither side
on no account
on no occasion
On no occasions, to Nietzsche
on no particular place
On notice
On now
on oath
On occasion
On occasion ... and how 20-, 25- and 43-Across go?
On occasion, once
On occasion, what Timmy used to call Lassie
on of as a representative
on off control
On offense, on a diamond
On on board
On one extreme
on one occasion
On one of these pages
on one s
On one side
On one's ___ (alert)
On one's ___ (independent)
On one's back
On one's belly
On one's case
On one's duff
On one's feet
On one's game