Some of the posts indicate that ヲ is very rare. This isn't really the case. In general Japanese usage, yes, it is very rare. However, if you have all-katakana text, then you will always find を written as ヲ.
All-katakana text might be encountered in child-oriented media such as video games. I have seen plenty of old games that use only katakana, such as Downtown Nekketsu Monogatari (known in the US as River City Ransom), Moero!! Junior Basket (known in the US as Hoops), and the original MSX/NES version of Metal Gear. ヲ is used in all of them. (Some of these games used all kana due to technical limitations, but sometimes all-kana text is a stylistic choice.)
In manga and video games, robots will often speak in katakana, much like using an all-caps "digital" font in English.
Transcriptions of morse code, telegrams, etc. will also use all-katakana text and therefore ヲ. So usage of ヲ is alive and well, it's just that the contexts where it's appropriate are infrequent except in certain media.