Short answer. Yes, it is an offense. Under Article 236 of the Revised Penal Code, anyone who assumes the duties and powers of a public office without first being sworn in, or without giving the bond required by law, commits "anticipation of duties of a public office" — punished by suspension from the office and a fine.
What the law says
Any person who shall assume the performance of the duties and powers of any public or employment without first being sworn in or having given the bond required by law
Revised Penal Code, Article 236 — Anticipation Of Duties Of Office. Read the full provision →
What the law says
shall be suspended from such office or employment until he shall have complied with the respective formalities
Revised Penal Code, Article 236 — Anticipation Of Duties Of Office. Read the full provision →
The offense of anticipating your duties
Article 236 punishes jumping the gun on a public office. It covers Any person who shall assume the performance of the duties and powers of any public or employment without first being sworn in or having given the bond required by law. In plain terms, you may have been appointed or elected, but you are not yet entitled to act until you have taken the oath and, where the law requires it, posted your bond. Starting to exercise the office before completing these formalities is what the article forbids — acting too soon, before you are properly installed.
The penalty is suspension and a fine
Unlike many articles, this one does not send the offender to prison. The consequence is that he shall be suspended from such office or employment until he shall have complied with the respective formalities, and he shall be fined. So the law's first aim is corrective: it stops the premature exercise of the office until the oath is taken and the bond given, then adds a monetary penalty. The peso range printed in the current text was set by Republic Act No. 10951 (2017); older copies of the Code show the smaller 1930 figures, so check which version you are reading.
Why the oath and bond come first
The requirement is not a mere formality for its own sake. The oath binds the officer to faithfully discharge his duties, and the bond, where required, protects the public against loss from his handling of funds or property. Article 236 enforces the sequence: qualify first, act second. It guards against someone wielding public authority before he has formally accepted its obligations and given the security the law demands. The wrong lies precisely in reversing that order — taking up the powers while the safeguards that are supposed to precede them are still missing.
What the article does not cover
Article 236 addresses acting before the oath or bond; it does not punish a mere delay in taking the oath by someone who has not yet started performing the duties. Nor does it reach an officer who has completed the formalities and is simply carrying out his office. It is also different from prolonging one's duties beyond the lawful period, which is a separate offense. The narrow target here is the premature assumption of official functions — the fix for which, the article itself says, is to comply with the formalities and only then act.