Pressure in OpneFOAM has two different dimensions in the solvers. In compressible solvers, the dimension is kg/ms^2 (N/m^2) and in incompressible solvers, it is normalized by constant fluid dnesity i.e. m^2/s^2.
Regarding the absolute pressure or relative pressure, it also dependent to the type of the solver. For incompressible solvers, the relative pressure is used and for compressible solvers, absolute pressure.