Merriam-Webster's Collegiate Dictionary:
ross dependency
ross ice shelf
ross sea
rosse
rossetti
rossi
rossini
rossinian
rossiya
rostand
rostellar
rostellum
roster
rostock
rostov-na-donu
rostov-on-don
rostral
rostrally
rostrate
rostrum
roswell
rosy
rosy periwinkle
rot
rota
rotameter
rotarian
rotary
rotary cultivator
rotary engine
rotary-wing aircraft
rotatable
rotate
rotation
rotational
rotative
rotatively
rotator
rotator cuff
rotatory
rotavirus
rotblat
rotc
rote
rotenone
rotgut
roth
rotherham
rothesay
rothko
rothschild
roti
rotifer
rotisserie
roto
rotogravure
rotor
rotorcraft
rotorua
rototill
rototiller
rotten
rotten borough
rottenly
rottenness
rottenstone
rotter
rotterdam
rotterdammer
rottweiler
rotuma
rotund
rotunda
rotundity
rotundly
rotundness
roturier
rou?
rouault
roubaix
rouble
rouen
rouge
rouget de lisle
rough
rough bluegrass
rough breathing
rough cut
rough endoplasmic reticulum
rough fish
rough it
rough lemon
rough rider
rough trade
rough-and-ready
rough-and-tumble
rough-dry
