is it 'tug a war' or 'tug of war' or both?

[what..the?]

thanks

[bednobs]

of

[maclarencat]

of

[steg]

of,.......... but up here it is tug o' war

[what..the?]

oh crap just printed a high print quality document with the spelling 'a'

[puternut]

I believe it is Of but generally 'shortened' to Tug O' War