It is defined in the U.S. Aeronautical Information Manual, but that hardly satisfies with looking in the context of international aviation. If you trace through a few ICAO documents and a U.N. treaty, you can come up with an answer.
— James Albright
Of course the answer may not be very useful for deciding what you want to do. It is more complicated that it should be.
If you are looking for the definition of "oceanic" you are it appears to depend on line-of-sight communications...
Airspace over the oceans of the world, considered international airspace, where oceanic separation and procedures per the International Civil Aviation Organization are applied. Responsibility for the provisions of air traffic control service in this airspace is delegated to various countries, based generally upon geographic proximity and the availability of the required resources.
Source: AIM, Pilot Controller Glossary] OCEANIC AIRSPACE
Airspace over the seas where line-of-sight communications or ATC surveillance (via radar or ADS-B) are not available. Air traffic control is provided using procedural control and procedural separation in accordance with ICAO.
Source: AC 91-70B, Appendix A, ¶A.2.5.1, Oceanic Airspace
If you are looking for "overwater," that's a little more easily defined...
Survival equipment for overwater operations.
(a) No person may take off an airplane for a flight over water more than 50 nautical miles from the nearest shore unless that airplane is equipped with a life preserver or an approved flotation means for each occupant of the airplane.
(b) Except as provided in paragraph (c) of this section, no person may take off an airplane for flight over water more than 30 minutes flying time or 100 nautical miles from the nearest shore, whichever is less, unless it has on board the following survival equipment:
(1) A life preserver, equipped with an approved survivor locator light, for each occupant of the airplane.
(2) Enough liferafts (each equipped with an approved survival locator light) of a rated capacity and buoyancy to accommodate the occupants of the airplane.
(3) At least one pyrotechnic signaling device for each liferaft.
(4) One self-buoyant, water-resistant, portable emergency radio signaling device that is capable of transmission on the appropriate emergency frequency or frequencies and not dependent upon the airplane power supply.
(5) A lifeline stored in accordance with §25.1411(g) of this chapter.
(c) A fractional ownership program manager under subpart K of this part may apply for a deviation from paragraphs (b)(2) through (5) of this section for a particular over water operation or the Administrator may amend the management specifications to require the carriage of all or any specific items of the equipment listed in paragraphs (b)(2) through (5) of this section.
(d) The required life rafts, life preservers, and signaling devices must be installed in conspicuously marked locations and easily accessible in the event of a ditching without appreciable time for preparatory procedures.
(e) A survival kit, appropriately equipped for the route to be flown, must be attached to each required life raft.
(f) As used in this section, the term shore means that area of the land adjacent to the water that is above the high water mark and excludes land areas that are intermittently under water.
Source: 14 CFR 91, §91.509
Communication and navigation equipment for overwater operations.
(a) Except as provided in paragraphs (c), (d), and (f) of this section, no person may take off an airplane for a flight over water more than 30 minutes flying time or 100 nautical miles from the nearest shore unless it has at least the following operable equipment:
(1) Radio communication equipment appropriate to the facilities to be used and able to transmit to, and receive from, at least one communication facility from any place along the route:
(i) Two transmitters.
(ii) Two microphones.
(iii) Two headsets or one headset and one speaker.
(iv) Two independent receivers.
(2) Appropriate electronic navigational equipment consisting of at least two independent electronic navigation units capable of providing the pilot with the information necessary to navigate the airplane within the airspace assigned by air traffic control. However, a receiver that can receive both communications and required navigational signals may be used in place of a separate communications receiver and a separate navigational signal receiver or unit.
(b) For the purposes of paragraphs (a)(1)(iv) and (a)(2) of this section, a receiver or electronic navigation unit is independent if the function of any part of it does not depend on the functioning of any part of another receiver or electronic navigation unit.
(c) Notwithstanding the provisions of paragraph (a) of this section, a person may operate an airplane on which no passengers are carried from a place where repairs or replacement cannot be made to a place where they can be made, if not more than one of each of the dual items of radio communication and navigational equipment specified in paragraphs (a)(1) (i) through (iv) and (a)(2) of this section malfunctions or becomes inoperative.
(d) Notwithstanding the provisions of paragraph (a) of this section, when both VHF and HF communications equipment are required for the route and the airplane has two VHF transmitters and two VHF receivers for communications, only one HF transmitter and one HF receiver is required for communications.
(e) As used in this section, the term shore means that area of the land adjacent to the water which is above the high-water mark and excludes land areas which are intermittently under water.
(f) Notwithstanding the requirements in paragraph (a)(2) of this section, a person may operate in the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean west of a line which extends from 44°47'00" N / 67°00'00" W to 39°00'00" N / 67°00'00" W to 38°30'00" N / 60°00'00" W south along the 60°00'00" W longitude line to the point where the line intersects with the northern coast of South America, when:
(1) A single long-range navigation system is installed, operational, and appropriate for the route; and
(2) Flight conditions and the aircraft's capabilities are such that no more than a 30-minute gap in twoway radio very high frequency communications is expected to exist.
Source: 14 CFR 91, §91.511
If you comb through the ICAO regulations, you come up only with this:
Oceanic flights are conducted in airspace where no sovereign rights are exercised and where normally, in that airspace, more than one State is concerned with the provision of ATS.
Source: ICAO Doc 9426, Part II, §4, Chapter 1, ¶1.1
So oceanic airspace begins where a State's airspace ends. But what constitutes a State's airspace?
Territory of a State
For the purposes of this Convention the territory of a State shall be deemed to be the land areas and territorial waters adjacent thereto under the sovereignty, suzerainty, protection or mandate of such State.
Source: Convention on International Civil Aviation, Article 2
Suzerainty: a sovereign or state having some control over another state that is internally autonomous.
So a State's territory includes its land areas and adjacent territorial waters. But what are territorial waters?
Every State has the right to establish the breadth of its territorial sea up to a limit not exceeding 12 nautical miles, measured from baselines determined in accordance with this Convention.
Source: United Nations Convention on the Law of the Sea, §2, Article 3, Breadth of the territorial sea
Of course not every signatory has agreed to this and many claim less and many claim more. But generally speaking, the answer is 12 nm from land. From this we can conclude that oceanic airspace normally begins 12 nm from the borders of States bordering the oceans.
14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation
Advisory Circular 91-70B, Oceanic and International Operations, 10/4/16, U.S. Department of Transportation
Aeronautical Information Manual
Convention on International Civil Aviation Done at Chicago on the 7th Day of December 1944
ICAO Doc 9426 - Air Traffic Services Planning Manual, International Civil Aviation Organization, First (Provisional) Edition, 1984