The Ideal Gas Law is usually written PV=nRT, where R=8.314 SI units. n is the number of moles, so we need the molecular mass of O₂=2(15.9994)=31.9998, which we’ll round up to 32, so n=196/32=6.125 approx.
56°C=273.15+56=329.15°K.
V needs to be in cubic metres, so 26.1L=26100ml=26100cc=26100/1000000=0.0261 cubic m.
Therefore 0.0261P=(6.125)(8.314)(329.15)=16761.4, so P=16761.4/0.0261=642199 pascals=642.2 kilopascals.