They opened the scoring after nine minutes with a try from Fox, converted by Moir (0-7).
Moir then kicked a penalty for 0-10, but after 28 minutes Sandal got back in the match at 7-10 with a converted try of their own.
Just six minutes later and it was all square at 10-all when the home side kicked a penalty.
That was the way it remained to half-time, but another penalty from Moir saw Alnwick back in front at 10-13.
Scrum half O’Reagan then went over for a second Alnwick try, unconverted, after 67 minutes, which put Alnwick further in front at 10-18.
Then on 78 minutes Paul added a third try, converted by Moir, which put the visitors in control at 10-25.
Alnwick now look forward to their first home league game of the season against Billingham on Saturday (September 11).