Thanks Michael I shall try to explain my thinking, it may be a bit waffly so apologies for that!
The main reason is psychological. I know we don't count calories here and we eat until sated but I binge eat with poor impulse control around food and drink (I was an alcoholic for 7 years and it's the same sort of impulse behind both). If I ate as much as I wanted I wouldn't lose weight. I have got to the stage where I need to do that for health reasons because I won't be able to get away with it for ever and I am no spring chicken. (An added reason is because covid seems to affect overweight people more so I want to get leaner).
So I have been limiting myself to around 2200 cals a day with frozen veg, pasta and rice. I have lost about 10kg since April and that suits me, I want slower loss so I minimise the risk of loose skin.
However, I have become so fed-up of pasta and veg that I am in danger of massively undereating, meaning loss will be too rapid for my liking. I *really* like overnight oats and so eating that number of calories to enable a slow loss won't be a problem as I could eat it until it comes out of my ears.
In addition I need very cheap food at the moment as my finances are dire. Cheap pasta & frozen veg, now oatmeal, fit the bill. I am a somewhat eccentric, dare I say basic, man. I like to keep things simple.
Oatmeal will be the only starch source for the time being.
I won't be eating fruits but I will be eating a kilo(!) of frozen veg a day (made up of carrots, cauli, broccoli and peas) followed by the oats as a nice, appetising dessert to look forward to.