Spouse is on a 6-month extension (pending I-765 processing related to a I-485 application) of his expired F-1 OPT. Is he considered a resident or nonres alien for taxes?

He is considered a resident.  He was in the US for more than 5 years or part years on an F-1 visa(2013, 2014, 2015, 2016, 2017).  He would be considered a resident in 2018 as of January 1, 2018.  That is the date he would start counting his days of presence for the substantial presence test.

 To meet this test, you must be physically present in the United States (U.S.) on at least:

1. 31 days during the current year, and
2. 183 days during the 3-year period that includes the current year and the 2 years immediately before that, counting:

• All the days you were present in the current year, and
• 1/3 of the days you were present in the first year before the current year, and
• 1/6 of the days you were present in the second year before the current year.