“Hey Arnold!” voice actor and writer Steve Viksten died on Monday. He was 54.

He lent his vocal chops to the character of Oskar Kokoshka, a boarding house resident with an ambiguous Eastern European accent, for the hit animated Nickelodeon show. Viksten wrote most of the series’ episodes with creator Craig Bartlett, in addition to penning the show’s movie adaptation.

His writing credits also include episodes of “Rugrats,” “Recess,” “Duckman” and “The Simpsons.”

Bartlett paid tribute to Viksten with a picture of a hatless Arnold he posted on his Facebook account.

“Steve Viksten’s first solo script for ‘Hey Arnold!’ was 3A ‘Arnold’s Hat,’” he wrote. “It’s one of my favorites: funny and sad and cool, and we get our first glimpse into the mystery of his missing parents. Today Arnold is hatless as I remember Steve.”

In addition to “Arnold’s Hat,” he also wrote the popular “Hey Arnold!” ep “Stoop Kid” (remember the chant “Stoop kid’s afraid to leave his stoop!”?)

Here’s a clip of the formerly illiterate Oskar testing out his new reading skills after tutoring sessions with Arnold.