Does the years living in UK can be counted towards the OAS calculations? I know the fact that max OAS calculations require the 40 years residency in Canada.

no

I think Bats is right. I could never find anything definitive on it, just that general "time spent in other countries may count" and you might think that given there are some reciprocal arrangements between the two countries that it might.
But given time in the UK (National Insurance) may qualify one for a UK pension, it wouldn't seem right for it to count double if you see what I mean.

What I do know is that last year I qualified for partial OAS and it was very obviously based solely on my residency years in Canada.

This is covered in the Social Security agreement between the two countries. https://www.treaty-accord.gc.ca/text...=eng&id=102220

It seems to say that years in the Uk count if, while you were there, you were covered in Canada for CPP purposes.

OAS payments are based on your age and the number of years you lived in Canada after the age of 18, and it is subject to clawback depending on your world income, There is a benefit calculator on the site, you can register for a My Service Canada acct and get estimates for CPP as well as OAS.

https://www.canada.ca/en/services/be.../payments.html

Co-incidentally, I have just been discussing this with a Services Canada agent in relation to my CPP contributions. I worked in the UK for 20+ years before moving to Canada so I obviously have many years of National Insurance contributions (as an employee, not self-employed).
The call agent said when you complete CPP claim form (ISP-1000) there's a section to let Services Canada know which country you lived in, the period of time you lived there and your NI number.
He said it takes a long time to sort out but it's not possible to predict the effect on CPP entitlement but, he implied, UK contributions could improve your CPP.

So, my question is, does anyone have practical experience of this or can anyone quantify this? i.e. how would I know for sure whether my NI conts actually increase my CPP, and if so, by (approximately) how much?
And if my NI conts are being "used" to enhance CPP, would this have an adverse effect on my UK State Pension?

(Incidentally, I'm not sure I've been given correct information by the call agent as there's zero information I can find about this, so I thought I'd ask the experts on here! If anyone knows a good source of info to confirm/repudiate this, please let me know. I can't find anything authoritive on Canada.ca)

EDIT: after some more research, it seems that time working abroad only counts towards qualifying periods not rates.
​​​​​​⠀‹https://www.canada.ca/en/services/benefits/publicpensions/cpp/cpp-international/eligibility.html#gc-document-nav

If you have time use this YouTube link

https://www.youtube.com/watch?v=OJk8Ahs2KOU

https://youtu.be/OJk8Ahs2KOU

I am Canadian - by age 60 I have worked 20 years in Canada and almost 20 in the UK, I will retire in Canada, My Canadian CPP Retirement will be paid based on the contributions I made while in Canada. At age 65 I will get my Canadian OAS based on my Canadian years of residency and at 67, I will get my UK State Pension will be paid based on my UK contributions. There is no tax treaty for the UK for CPP as such but if you worked in the UK for a Canadian employer DND etc it is different.

You can sign up online for a Service Canada acct and see your CPP and OAS forecast based on your personal Canadian contributions
https://www.canada.ca/en/employment-...y-account.html

More info here
https://www.canada.ca/en/services/be...d-kingdom.html

You can't buy back Canadian contributions however you may be able to buy back NI contributions. I have recently bought back UK NI contributions which will almost double my State Pension at 67.
https://www.gov.uk/voluntary-nationa...-contributions