Oxygen-15 has 7 neutrons and 8 protons.
It is unstable because it is below the band of stability.
Its n:p ratio is 7:8 or 0.88:1.
It has too few neutrons for the number of protons, but it would become more stable if it could gain a neutron or lose a proton.
One way to do this is by positron (β⁺) decay.
β⁺ decay accomplishes both of these.
It gets rid of a proton and forms a neutron (and a neutrino).
For oxygen-15, the reaction is