Onset and outset (both nouns) are apparently the most controversial English pair I’ve come across. I researched their similarities and differences for months and got some answers—both the confusing and the convincing. Despite that, I’ve found a common solution from reliable sources on contemporary English.

To avoid being sentimental, let’s look at a few of the answers I found on English language forums across different continents.

1. Onset describes the beginning of something that will continue; outset describes the beginning of something that has not been experienced before.
2. Onset indicates that something is forming; outset indicates that something has fully formed.
3. Onset refers to the time a symptom appears; outset refers to the time something actually begins.
4. Onset talks about the beginning of something not under human control (diseases, earthquakes); outset talks about the beginning of human activities.

The above definitions are all different and seem not to have anything in common, making it quite difficult for us to get our heads around them.

However, I found the solution after consulting the Oxford Dictionary, Cambridge Dictionary, Macmillan Dictionary, Collins Dictionary, British National Corpus, Corpus of Contemporary American English, and the British Council.


Onset
Onset is the start or beginning of something, especially something unpleasant.

Its etymology can be traced to the 1530s when it was used as ‘attack, assault’. By the 1860s, it had evolved to ‘beginning, start’, and was later (1580s) used to refer to ‘disease, calamity’. (etymonline.com)

Examples:

1. The onset of winter is dreadful to people who catch flu easily.
2. I’d say the onset of HIV began another phase in human existence.
3. The onset of asthma is usually very critical.
4. We can prevent the onset of flood if we dredge the canal.

Other examples include the onset of strike, revolution, infection, destabilization, crisis, pain, fear.

Note that it can also be used to state when or how a medical condition started, e.g. sudden-onset stroke.


Outset
Outset, however, is simply the start or beginning of something. It is usually preceded by ‘at/from the’.

The earlier word (1670s) was outsetting; by 1759, outset became ‘act of setting out on a journey, business, etc.’ (etymonline.com)

Examples:

1. I knew from the outset she would agree to marry me.
2. He told us from the outset that the job is temporary.
3. At the outset of the project, a million naira was budgeted.
4. Young politicians spend more money on elections at the outset than when they become statesmen.

Generally, onset and outset mean ‘beginning or start’, but if we want to be more direct in distinguishing between a normal/positive and negative event or situation, we should use onset and outset as appropriate.

I hope this helps.
http://englishnaija.com/

Thanks Man, Am Guilded

Cool
However many people do not like the word "outset".