O'Connor, a 67-year-old former infantryman who fought in the Vietnam war, said he had one message for his president, who arrived in Danang Friday for an APEC leaders' meeting.
"Thank you for making America great again and taking our country back," O'Connor told AFP.
He said he was ready to brave the searing sun and heavy security along the beach where US marines first arrived more than 50 years ago to support the fight against communist-backed Viet Cong soldiers.
"If I get lucky I might be able to see Donald Trump," O'Connor said, after a long flight from South Dakota.
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Sadly, he left his 'Make America Great Again' hat back home, but planned to don a red replacement on his mission to see the man whose politics have him captivated.
He expressed hope that Trump would pledge help to the victims of Agent Orange who still live with crippling disabilities after being sprayed with the toxic defoliant during the bloody and bitter war.
The US government is working with Vietnam to clean up dioxin left over from the war.
O'Connor frequently travels to Vietnam to provide bicycles to school children in remote communities, a far cry from his mission during the war.
"We used to kill these people," he said remorsefully.
Trump's own service record -- he deferred military service several times, including once reportedly for bone spurs -- doesn't bother Oconnor.
As for Trump's controversial comments about former vet John McCain, who he has said isn't a hero because he was captured during the war, O'Connor chalks them down to a personal grudge -- and Trump's penchant for off the cuff remarks.
"He's not politically correct, he says some things sometime that he should not," O'Connor said.