if you are able to characterize the oil you can calculate isothermal compressibility k = -(1/V)*dV/dp with a equation of state as SRK or PR (my usual approach with tools as Prode Properties), or the bulk modulus (the inverse of compressibility) as suggested by zdas04, differently you could use generic correlations, see for example Vazquez and Beggs correlations for fluid physical property prediction or Al-Marhoun the coefficient of isothermal compressibility of black oils, there are many different methods and I can't recommend one.