Organisation or Organization?

Spelling differences in some words are due to their British or American origin. You can check a list with the most frequent cases:

BRITISH	AMERICAN
-ise/ -yse	-ize/ -yze
Organise	Organize
Organisation	Organization
Analyse	Analyze
Realise	Realice
Recognise	Recognize
Apologise	Apologize
Privatise	Privatize
Privatisation	Privatization
Practise (practice in UK English is the noun)	Practice
-our	-or
Colour	Color
Harbour	Harbor
Favourite	Favorite
Neighbour	Neighbor
Honour	Honor
Behaviour	Behavior
-re	-er
Centre	Center
Litre	Liter
Theatre	Theater
-ence	-ense
Licence	License
Defence	Defense
Offence	Offense

VERBS ENDING IN-EL IN PAST TENSE
-elled	-eled
Travelled	Traveled
Cancelled	Canceled

Remember English spelling maintains the Latin or Greek origin while American spelling has been adapted to the way the word is pronounced.

According to Oxford Dictionary, there are some cases in which both ways are accepted, especially regarding the -ize endings.