Oslo Airport, Gardermoen, in Oslo, Norway.

What is the difference between OSL and ENGM?

OSL is the IATA code, used commercially — tickets, timetables, luggage tags. ENGM is the ICAO code, used operationally by pilots and air traffic control, and assigned geographically by region.

What airport is OSL?

OSL is

Oslo Airport, Gardermoen

Oslo, Norway

Why the code is OSL

The first three letters of Oslo.

The airport authority or a contemporaneous record states this directly.

The airport

IATA code: OSL
ICAO code: ENGM
Airport: Oslo Airport, Gardermoen
City: Oslo, Norway
Code origin: Documented

Gardermoen was a military airfield and cavalry drill ground long before it was Oslo's main airport, which the ICAO code ENGM records. The terminal is built largely of Norwegian timber, which was a structural decision rather than a decorative one.

The other code, ENGM

Every airport has two codes. The three-letter IATA code, OSL, is the one on your boarding pass and your luggage tag, and it is assigned for commercial use. The four-letter ICAO code, ENGM, is the one pilots and controllers use, and it is assigned geographically: the first letter or two identify the region, so every airport in the same part of the world shares a prefix. That is why ENGM looks nothing like OSL.

Common questions

What airport is OSL?: Oslo Airport, Gardermoen, in Oslo, Norway.
Why is Oslo's airport called OSL?: The first three letters of Oslo.
What is the difference between OSL and ENGM?: OSL is the IATA code, used commercially — tickets, timetables, luggage tags. ENGM is the ICAO code, used operationally by pilots and air traffic control, and assigned geographically by region.

Norway Capital, area, population and currency
The flag of Norway Colours, symbols and lookalikes
Airport codes in Norway 1 here

Sources

OurAirports — OurAirports (Public domain)

How these figures are compiled and checked