It has been pointed out previously in the comments section of The Crimson that such an act of divestment would be only symbolic. The reason is that publicly traded shares only fund a company in its initial sale during the IPO (which, for oil companies, was decades ago at the very least). All subsequent trades of shares only impact valuation of the company. Harvard's divestment would take down the price of an oil company stock by a fraction of a fraction of a dollar* which means any purchases or sales of assets using shares in the near term subsequent to this divestment would require a fraction of a fraction more shares, which, in the grand scheme of things is meaningless.
*in all likelihood the investment managers would try to divest in portions to minimize losses, so the price movement might be even closer to zero.