Long historical note comparing major scores at WH and Oakmont; please bypass if this bores you.
Matt probably has the better of the argument here regarding "toughness," if that is measured by performance by winners and contenders relative to par in recent majors held at each course. Comparison:
Oakmont:
'94 USOpen: -5 wins (five other players under par); Roberts shoots 64, three shoot 65, 66 is shot once.
'83 USOpen: -4 wins (two other players under par); Nelson goes 65 (lowest round that day by 4 strokes!) and 67 (one of only six rounds that day in the 60s) on the weekend, a feat arguably more impressive than Miller's singular 63.
'78 PGA: -8 (three tied) wins (three other players under par); 66 shot three times; 67 shot five times.
'73 US Open: -5 wins (eight others under par, although six of those were at -2 or -1); Miller goes 63, but Wadkins shoots a nearly as good 65 on the same day in the same conditions (he went out early similar to Miller); 65 shot twice; 66 shot once. (Miller and Wadkins' rounds were really that good -- only two other players broke 70 that final day.)
'62 USOpen: -5 wins (three others under par); played as par 72 (par 71 in majors after this one); not many low rounds -- 67 shot just once, and 68 shot just once.
'53 USOpen: -5 wins (runner-up at +1; everyone else +4 or more); Hogan's opening 67 lowest round of tourney by two shots; only the Slammer shoots a round in the 60s (69) for the entire tourney.
WF:
'06 USOpen: +5 wins (everyone else +6 or worse); I forget how many rounds were under par, but it wasn't many.
'97 PGA: -11 wins (three others under par); 65 shot twice; 66 shot six times, three by Love; 67 shot nine times.
'84 USOpen: -4 (two tied) wins (everyone else +1 or worse); one round of 66 and one round of 67 in the tourney; both Norman and Irwin shot three rounds in the 60s, and neither won.
'74 USOpen: +7 wins; third place was +10; one 67 and two 68s shot; only seven under-par rounds for the tourney.
'59 USOpen: +2 wins; two 67s and two 68s shot during tourney; no one under 71 (+1) in the final round.
Caveat and emptors: WF plays for majors at par 70; Oakmont at par 71, and par 72 in the '62 and '53 Opens, arguably giving pros one more chance per day for an under-par score (given that pros score lower on par 5s than any other holes). set-ups, conditions, and weather obviously vary. It's been fairly well documented (yes?) the USGA set up WF extraordinarily difficult in '74 as a response to Miller's 63 the year before.
In general, Oakmont appears to be more susceptible to some very low rounds, and appears somewhat easier to finish under par for the duration of the major.