The win gives McEwan (33) eight North Otago titles, including seven consecutive wins since 2008.
McEwan led from start to finish but made things difficult for himself in the final round with four bogeys in the first 10 holes.
However, he tidied up his round on the homeward run to finish with an even-par 72 to clinch the title.
A wedding and honeymoon meant McEwan missed the first two trials but since then he has lost the Cromwell event in a playoff, before winning at Queenstown and now at Oamaru.
There is a strong contingent trialling for Otago men's teams this year, but McEwan has sent a clear message he should retain the No 1 berth.
Dunedin professional Mahal Pearce was six shots behind McEwan while amateurs Cody McMullan (Chisholm Park) and Michael Ormandy finished second on 294.
Ormandy made an impressive start on Saturday but his third round of 79 cost him a chance of winning the event. In sharp contrast, McMullan scored 71 and 70 yesterday to rocket up the leaderboard to tie for second.
With a 3-under 69, McEwan took an early lead in the first round. He turned 1-under and made two birdies on his homeward nine. Ormandy relished playing at home and shared second place with Dunedin professional Mahal Pearce, one shot back on 70.
Ormandy turned 2-under and had seven pars, one birdie and one bogey on the back nine. Jeremy Morgan (Otago) made a good start in the event with an even- par 72 to lie in fourth place going on to the afternoon round.
The 47-strong field faced testing coastal winds in the afternoon with Sean Masters (Otago) posting the only sub-par round with 71 while Nicholas Borren (Otago) shot 72.
But it was steady McEwan who increased his lead to two at the halfway stage with a 1-over 73.
Ormandy and Masters shared second place with an even-par aggregate for the day while Borren was third on 146.
In the afternoon there were only six players on 150 or better while some scores soared in the difficult scoring conditions.
McEwan took control on Sunday morning with another 73, opening up a six-shot lead on his nearest rivals.
Borren, Pearce and Masters shared second place on 221 while Ormandy was in fifth place two further shots back.
McMullan had the lowest score in the third round with 71 to leap up to sixth while Daniel Elder and Kris Rush (both Otago) improved their positions with par rounds.
The next trial is the Southland matchplay next weekend but, after six tournaments, few Otago players are expected to play.
