I'm going to ask this again: What is the point of divestment?
If it is to send a message to the oil companies, they're not going to hear it. In fact, they'll never even know the divestment happened unless they read a press report. If Oxford does, in fact, sell its shares of oil and gas companies, it will do so on the open market at established market rates. That means that some other investor will purchase the shares from Oxford, with no involvement whatsoever by the oil company. After the oil company created the shares and sold them the first time, they're completely out of the process of subsequent trades.
And as far as obligations go, the university does have at least a moral obligation to those alumni whose contributions helped create the endowment. The university is obligated to make the best use of those monies it can, including investing prudently for both growth and protection.
Refusing to purchase high yield investments to send a political message that will not be heard would be highly irresponsible.