It has more protons pulling on its electrons. Lithium has 3 protons pulling on its one valence electron (for now I'm going to treat all electrons in the 2s/2p energy level as being equidistant from the nucleus for simplicity), while Oxygen has 8 protons pulling on each electron (ignoring charge screening). So just based on this, you'd expect oxygen to pull on its electrons a little under three times as hard as Lithium (8/3x as hard). Now, since Oxygen has that much more charge pulling those electrons in, they're pulled in a lot closer than that are in Lithium (60pm for 0, 152pm for Li), about 2.5x times closer. Since the force on charged particles is proportional to the product of the charges over the distance squared, you can see Oxygen has a much stronger grip on its valence electrons than Lithium does. To put some (very appropriate) numbers to it, Oxygen's pull is proportional to 8/60^2 = .0022, and Lithium's pull is 3/152^2 = .00013. So very roughly the force on the electrons in O are about 17x higher than the electrons in Li. It's a lot more complicated than this,
but this is the basics of it.
