The oak measures 3.5 m in diameter and 13 m in girth at its widest part; being 2.8 m. and 9.58 m at waist level. The oak reaches 23 m in height with only side branches remaining alive. It is believed to be at least 1,500 years old, possibly, as much as 2,000 years old; this makes it the oldest oak in Lithuania and one of the oldest in Europe. However, exact age measurements are difficult, as the inner part of the trunk has been removed.
The oak has been protected since 1960. In 1987 it was listed as state natural monument of republican significance of the Lithuanian SSR. In 2000 the oak was declared a natural monument and included in the list of protected objects in Lithuania and in 2003 it was declared the botanical natural heritage site.
The oak is heavily damaged by various infections, fungi and overgrown with moss. The branches are supported. Although in early March 2021 new supports were installed, one branch broke on March 31 2022. Since 2005 the state of the oak was researched and maintained by Czech arborist Martin Nemec. He identified major threats and suggested a long-time maintenance plan.
An offspring from the acorns of Stelmužė Oak grows in the National Revival Oak Grove (Tautinio atgimimo ąžuolynas) by Jonas Basanavičius's native homestead and museum,  together with the offspring of other famous Lithuanian oaks. 
See also Edit
- Gamtos paveldo objektai tvarkimas (Management of Natural Heritage Objects)
- Stelmužės ąžuolas, Universal Lithuanian Encyclopedia
- "Stelmužė oak". VisitZarasai. Retrieved 2021-09-28.
- Daugiau nei tūkstantį metų skaičiuojantis senolis Stelmužės ąžuolas – mokslininkų akiratyje, lrytas.lt, March 5, 2010
- Evaldas Vylius Navys, Stelmužės milžinas – jau ne galiūnas, delfi.ee, August 22, 2008
- Jono Basanavičiaus gimtinė ir Lietuvos tautinio atgimimo ąžuolynas