If a local arborist, infestation inspector, or other official has marked your tree with a condemnation, removal, infestation, or other symbol to tamper, attempt to remove or obscure it could be both a civil and criminal act.
You asked for legal advise - this would indicate to me that either the mark was placed by an official indicating disease or infestation, or unsafe condition of the tree and/or possibly that the tree or part of it resides on an easement and encroaches on present or future use.
This is not a legal forum, however, be advised if the paint (like a red or orange X, for example) was placed by an official act - tampering with it could bring penalty.
Now....on the other hand if your reason for asking for legal advice is that you or your child committed a vandalism upon said tree and you now wish to restore the tree with a minimum of damage - you'll need to indicate more than you have; AND suggest you consult a professional arborist and pay for the restoration, SINCE if you committed a vandalism, AND the tree suffers damage as a result of your clean-up efforts (and/or you are the subject of a civil tort) you'd want to mitigate not only the paint damage but the potential damage during removal of same - and spread the liability around regarding if the tree suffers additional damage or stress (and/or has to be removed or replaced).
You didn't mention the age of the oak, nor the type. Dealing with a swamp oak versus a cork oak versus an established burr, white or red oak, versus a black oak would be entirely different. Also the extent of the paint - type of paint, etc. The age and health of the tree would also effect what might be the most prudent approach.