I guess this info is also on the sleeve of IMD album but anyway. Went to this site, typed in "xylophone" and there were several xylophone sounds. One of them sounded like the one on Omen, and in fact it is already played from the same note as in the track. So Liam wanted a xylophone sound and simply got it by typing it in and taking the sample?
This is true. The source was not credited in the sleeve of the first release of Invaders Must Die, but was later added to the IMD special edition sleeve, and WOF credits.