Whisk the oat milk and cornstarch in a measuring cup or bowl until smooth. You want to remove any small clumps of cornstarch before you heat it.
To a medium pot, add the sugar and oat milk/cornstarch mixture.
Turn the heat to medium-high and whisk until smooth. Bring to a simmer and continue whisking for about 5 minutes, or until the mixture gets hot enough to thicken.
Remove from the heat and stir in the vanilla. If you end up with any clumps of cornstarch, use an immersion blender and blend until very smooth. You could also blend in a high-powered blender if needed.
Place in a bowl and cover with plastic wrap to avoid a film forming on the top. Place in the refrigerator to cool for about 2 hours.
Once the custard has cooled, remove it from the refrigerator and add the Coco Whip. Use a electric mixer to mix it in.
Line the bottom of a trifle dish or 2-2.5 quart dish with 1/3 of the cookies, then 1/3 of the banana slices, and spread 1/3 of the custard on top of the bananas. Repeat the layers two more times, then garnish with more cookies or cookie crumbs plus more bananas on top, if desired.
Cover and let chill in the refrigerator for at least 4 hours if possible, before serving. This will let the flavors mingle and help the cookies soften a bit.
Serve immediately. Store leftovers covered in the refrigerator for 3-4 days. Unless you use super ripe bananas the bananas won't brown too much. Enjoy!