I wouldn't rule out OT then. By the time they get to be toddlers, the same OT signs (crying, unhappiness, punchiness) don't always show up. My son has never shown a single ounce of being tired before bed. Not once. Not even when he skips a nap. But believe me, we STILL get NWs if he's OT at bedtime. I have to go by A time to this day!
I think I'd make bedtime earlier and see if that works. If nothing else, if he plays in bed then at least he'll fall asleep within his sleep window which will give you a better chance of him sleeping better/longer. 6.5 hours is just so very very long for an 18 month old. 5 hours is more appropriate. A 6:30 bedtime would be great, or 7 at the latest.
The other thing I might try, to be honest, is move that nap a bit later. You've got only 4.5 hours A time BEFORE nap. That could be why nap is only 90 minutes -- he may not quite be tired enough. I might move nap to 12:30 and see if that helps him sleep a bit longer. Worst case he still sleeps 90 minutes but that pushes you out to 2 which makes a 7 pm bedtime workable.
The playing around at bedtime is a CLASSIC OT sign in toddlers. Totally classic. I think a bedtime more in keeping with his A time capabilities will cut the playing down a lot, possibly to none!