Hydroxide as leaving group in Aldol Condensation in base
Quoting ORGANIC  CHEMISTRY by Jonathan Clayden  ,Nick Greeves , Stuart Warren ,these are elimination reactions .You cannot normally eliminate water from an alcohol in basic solution as hydroxide is a bad leaving group.It is the carbonyl group that allows elimination here: these are E1cB reactions, with a second enolization allowing the loss of OH−.

the base-catalysed aldol reaction sometimes
gives the aldol and sometimes the elimination product. The choice is partly based
on conditions—the more vigorous the conditions (stronger base, higher temperature,
longer time) the more likely elimination is to occur—and partly on the structure of the
reagents.

The key to what is going on is the carbonyl group.Negative
charges are stabilized by conjugation with carbonyl groups,The proton that is removed in this
elimination reaction is adjacent to the carbonyl group, and is therefore also rather acidic (pKa about 20).The anion that results is stable enough to exist because it can be delocalized on to the carbonyl group.


This next step is ,


The leaving group is not lost from the starting
molecule, but from the conjugate base of the starting molecule, so this sort of elimination,
which starts with a deprotonation, is called E1cB (cB for conjugate base).