A buffer reaction actively resists changes to the concentration of a chemical species. Typically, buffering reactions have only been able to regulate the concentration of hydronium (i.e., pH) and other ions. Here, we develop a new class of buffers that regulate the concentrations of short sequences of DNA (i.e., oligonucleotides). A buffer's behavior is determined by its set point concentration, capacity to resist disturbances, and response time after a disturbance. We provide simple mathematical formulas for selecting rate constants to tune each of these properties and show how to design DNA sequences and concentrations to implement the desired rate constants. We demonstrate several oligonucleotide buffers that maintain oligonucleotide set point concentrations between 10 and 80 nM in the presence of disturbances of 50 to 500 nM, with response times of less than 10 min to 1.5 h. Multiple buffers can regulate different sequences of DNA in parallel without crosstalk. Oligonucleotide buffers could stabilize and restore reactant concentrations in DNA circuits or in self-assembly processes, allowing such systems to operate reliably for extended durations. These buffers might also be coupled to other reactions to buffer molecules other than DNA. In general, an oligonucleotide buffer can be viewed as a chemical "battery" that maintains the total chemical potential of a buffered species in a closed system.