That section of the Old San Antonio Road is served by Texas State Highway OSR.

On August 2, 1943, the western terminus had been extended to end in San Marcos, along its current route, replacing part of SH OSR.

SH 21-H) is a 1.362-mile (2.192 km) long business route that runs through Kurten in central Texas.

SH 21-P) is a 4.170-mile (6.711 km) long business route that runs through Nacogdoches in eastern Texas.

[13] The route was formed from an old section of SH 21, when the main highway was re-routed onto Loop 224 and US 59 on October 25, 2018.