Working on F1-OPT in 17/18 in the US, thought I file a 1040NR, Sprintax tells me I am a resident. 2018 255 days (F1-OPT), 2017 326 days (F1-OPT), 2016 160 days (F1)

Yes, you would be considered a resident for tax purposes in 2018 because you meet the substantial presence test for the calendar year. You are only exempt for 5 years on the F1 visa after that the substantial presence test determines if you are a resident for tax purposes. 

But the substantial presence test says F1's are exempt. Regarding your comment around the 5 years, why wasn't I considered a resident in 2017 then? That is confusing to me because I was categorized by H&R Block as non-resident for tax purposes in 2017 when they did my taxes last year. I am waiting for you feedback on this and will use your services going forward. (Note: I had 3 different F1's - First one 2009 to 2012 (Bachelor Degree), Second 2013 (summer program), Third 2016 to 2018 (MBA Degree and F1-OPT)

But the substantial presence test says F1's are exempt. Regarding your comment around the 5 years, why wasn't I considered a resident in 2017 then? That is confusing to me because I was categorized by H&R Block as non-resident for tax purposes in 2017 when they did my taxes last year. I am waiting for you feedback on this and will use your services going forward. (Note: I had 3 different F1's - First one 2009 to 2012 (Bachelor Degree), Second 2013 (summer program), Third 2016 to 2018 (MBA Degree and F1-OPT)

You are exempt for 5 years on a F1 visa. This is a lifetime exemption, so once you used up the 5 years, you cannot be exempt again on a F1. Therefore after the 5 years, the substantial presence test starts. You should have been a resident for tax purposes in 2017. Please see these examples on the IRS website: <a rel="nofollow" target="_blank" href="https://www.irs.gov/individuals/international-taxpayers/alien-residency-examples">https://www.irs.gov/individuals/international-taxpayers/alien-residency-examples</a>