You purchase a flag and decide to burn it, you’re golden. You take a flag that’s not yours and burn it, you have committed a crime.
Note that the Oklahoma state law also covers defacement of property (i.e., graffiti, stickers, etc.):
Title 21. Crimes and Punishments
§21-1760. Malicious injury or destruction of property generally - Punishment - Damages.
Universal Citation: 21 OK Stat § 21-1760 (2014)
A. Every person who maliciously injures, defaces or destroys any real or personal property not his or her own, in cases other than such as are specified in Section 1761 et seq. of this title, is guilty of:
1. A misdemeanor, if the damage, defacement or destruction causes a loss which has an aggregate value of less than One Thousand Dollars ($1,000.00);
2. A felony, if the damage, defacement or destruction causes a loss which has an aggregate value of One Thousand Dollars ($1,000.00) or more; or
3. A felony, if the defendant has two or more prior convictions for an offense under this section, notwithstanding the value of loss caused by the damage, defacement or destruction.
B. In addition to any other punishment prescribed by law for violations of subsection A of this section, he or she is liable in treble damages for the injury done, to be recovered in a civil action by the owner of such property or public officer having charge thereof.