I'm planning a trip to London in early October and considering a day trip to Oxford or Cambridge. The fall (Michaelmas) terms for both universities start in early October (the 7th and 8th). Would a visit be more difficult due to students arriving? Would the colleges be accessible to visitors?
They will be busier and porters will be more harassed, but no less accessible to the public than usual. (Many colleges are not that accessible to the public, they are working educational institutions not tourist attractions.) Ones with a big tourist potential like Christ Church will still be open.
The period where there is much more restricted access is May/June when undergraduate students are studying for and then taking the main exams.