Van Os says his doctor diagnosed him with PTSI and prescribed antidepressants.
"He was a former military doctor who was able to recognise the symptoms, which can be hard to spot.
"The condition has a spectrum and I am at the lower end of that range."
The medication, he says, helps to "unscramble the serotonin levels" in his brain.
"Talking about it helps immensely - especially with people who have been in combat zones.
"If you know someone who has been to war ask them what's happening and how they are feeling if they seem to be struggling with life."
After seeing the "guys who had it bad" during his years of service, Van Os says it had not occurred to him that his own symptoms could be PTSI.
Van Os retired from the British Army four years ago and came back to Whanganui to take up his role as reverend of St Andrew's Presbyterian Church in Glasgow St.
He left Whanganui as an 8 year-old to live with his family in Tokoroa and later studied in Otago before finding his calling.
Returning to a place of happy childhood memories has given him a sense of serenity and life coming full circle, he says.
"The Whanganui RSA have been brilliant in helping me to re-assimilate and members of the congregation have been very helpful as well."
It is very fitting, he says, that the theme for this year's RSA Poppy Appeal, not all wounds bleed, highlights the fact that mental health injuries are the most common, but least understood, of all wounds suffered by New Zealand servicemen and women.
New Zealand now has 41,000 veterans – the most at any time since the end of WWII and about three quarters of those have served since the Vietnam War.
RSA national president BJ Clark said the 2018 Poppy Appeal is aimed at providing recognition and support for younger veterans.
"This younger generation of veterans have to deal with many of the same life challenges of those earlier generations of servicemen and women but perhaps without the understanding of the public that they too had experienced some dangerous, stressful and personally distressing situations in their service for New Zealand."
Funds raised from the appeal will help to support former servicemen and women with the mental health challenges they are dealing with on a daily basis, he said.