Wai_wai,

 Computational complexity is measured either in running time or memory usage. If you read further in the link you have provided, it clearly says that big O only provides upper bound. Sorry, if I am making it confusing here, but whatever I wrote in my previous post is true. Big O(n) only gives the upper bound. 

Here is an open courseware from MIT that describe everything about algorithm complexities. http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-046JFall-2005/CourseHome/index.htm

Good luck.