These two Orthetrum species are very similar and can be confusing to distinguish.
O. sabina has an enormous global range including North Africa, Russia, the Middle East and S.E. Asia, extending through the Philippines and PNG to northern and eastern Australia. It just sneaks into our European theatre being in Turkey and Greece.
O. serapia is, happily for most of us, much more restricted in range being limited to the Philippines, PNG, northeastern Australia and islands around the adjacent Coral Sea. In all this range (I believe), it overlaps with O. sabina where confusion can occur.
The key to distinguishing these two cousins in the field is the complexity of the abdominal markings on S4.