Originally Posted by rch4u

... unless you're travelling on a Saturday.

Weekdays through the end of October:

UA531 ORD 9:55 AM OAK 12:23PM 752 4h28m
UA752 OAK 1:28 PM ORD 7:30 PM 319 4h02m

Saturday, October 28:

UA531 ORD 9:55 AM OAK 12:23PM 752 4h28m
UA752 OAK 1:33 PM ORD 7:32 PM 752 3h59m

No other 757s come into OAK around the noon hour, so it's 99.99% certain to be the same aircraft on Saturdays.