Soldier saluting
It means respecting another person.
"o" stands as 'head of soldier' and "7" indicates 'arm'.
It is mostly used in
War Thunder video game, especially in Realistic and
Simulator game modes. Sometimes it's formed as with a capitalized "O (letter)" and typed as
O7. And rarely with zero, 07.
(German
Pilot shot down Brit Pilot)
Brit Pilot: That was a nice kill. o7
German Pilot: Thanks o7