Ryan O'Neal performance here is much like the film, and one should look at it for what is which is the male side of the love story in this Love Story. Firstly and most importantly he does have a believable enough chemistry with Ali Macgraw, that was integral for the film. O'Neal and Macgraw show the evolution of their relationship very well. They begin as most Hollywood romances do being slightly antagonistic to one another. They get over this quickly though, and their instant mutual attraction is handled well enough.
Their relationship continues along well enough, and both are extraordinarily effective in showing how they mature as a couple, and become more connected with one another as time goes on, and their relationship goes further along. They certainly have just an abundance of strong scenes together because they always bring out the true emotions including the scene of the famous(or infamous) line after they had a fight. The line alone sounds incredibly stupid, and perhaps it is, but in context of the film it works, because of their relationship does happen to work.
O'Neal's performance also is slightly risky in that he does in fact play Oliver as a bit of a fancy boy in the manner he speaks, but it actually does work. Yes he is wimp, but hey Oliver is suppose to be a wimp. O'Neal never really forces his emotional manner, or suggesting the upper class breeding of Oliver, he just brings it about quite naturally. O'Neal whole performance is effective actually, even if it is not an overly complex performance.