Pilot officer is the lowest commissioned rank in the Royal Air Force

Royal Air Force

The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 and the air forces of many other Commonwealth

Commonwealth of Nations

The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...

 countries. It ranks immediately below flying officer

Flying Officer

Flying officer is a junior commissioned rank in the Royal Air Force and the air forces of many countries which have historical British influence...

. It normally denotes an officer who has elected to join as a non-graduate direct entrant officer, as those with degrees usually only serve a week at the rank after graduation from the RAF College Cranwell. Some newly commissioned officers hold the lower grade of acting pilot officer

Acting Pilot Officer

Acting Pilot Officer is the lowest commissioned grade in the Royal Air Force, being immediately junior to Pilot Officer. Unlike other RAF ranks which officers may hold in an acting capacity, Acting Pilot Officer is maintained as a separate grade. It normally denotes an officer who has recently...

.
It has a NATO ranking code of OF-1 and is equivalent to a second lieutenant

Second Lieutenant

Second lieutenant is a junior commissioned officer military rank in many armed forces.- United Kingdom and Commonwealth :The rank second lieutenant was introduced throughout the British Army in 1871 to replace the rank of ensign , although it had long been used in the Royal Artillery, Royal...

 in the British Army

British Army

The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...

 or the Royal Marines

Royal Marines

The Corps of Her Majesty's Royal Marines, commonly just referred to as the Royal Marines , are the marine corps and amphibious infantry of the United Kingdom and, along with the Royal Navy and Royal Fleet Auxiliary, form the Naval Service...

.