Omarion passed on the Legos and Tonka truck for his son's 2nd birthday ... opting for an art piece that set the singer back $18k.

The R&B star hired artist Madsteez to do a surreal painting of his 2-year-old son, Megaa ... a gift he presented at his kid's Kidchella-themed b-day party over the weekend in Granada Hills.

We're told Omarion reached out to Madsteez four days ago to make the 36" x 36" acrylic and spray paint piece -- which took 2 days to create.

As for the party ... the kids performed on a mini-stage and played in a bunch of bounce houses. There was also plenty of face painting.

Not too shabby.