I am sorry that you are having difficulties. A suggestion: Instead of regarding a person who has lunch with you as "some kind of random", see that person as a friend. Talk to that person about his or her interests and ideas. Suggest further social activities. Think of friendship as a non-transactional interchange, not as centered on you or on the other person. Nobody is a "random". That person has needs just like you. Perhaps he or she is lonely and might like to spend time with you more often. Friendship requires effort from both friends if it is to grow.
Friendships at university evolve, and the second term is not usually as heady as the first. There is an old joke that people spend the second term shrugging off the friends they made in the first term. That can be partly true, but doesn't have to be.
Being alone to study is normal. People may be knuckling down to the work. Some have Mods this term. Others may be thinking of exams next term. If you treat being an undergraduate as your job, you can organise your time for work, rest, activity, and socialising. Talk to your tutor, a finalist, or a graduate student who was an undergraduate at Oxford about organising your time. Your youngest tutors may have been undergraduates not so many years ago.
Maybe start a new activity. Learn a new sport or hobby, take up a language class, join a choir, master a new dance technique, audition for a play, and so on.
If you are the same student who has posted elsewhere about drinking a lot before seeing a male friend at another college, try to drink less, or maybe not at all. If drinking is social and fun, great. If it's medication, stop. Talk to your "situationship" friend about how you feel.
Also, talk to the welfare staff at your college.
I hope that things pick up for you. Oxford is still the same Oxford that you enjoyed last term. Your first Oxford summer lies not far ahead.