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Oganesson (₁₁₈Og) is a synthetic element created in particle accelerators, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first and only isotope to be synthesized was ²⁹⁴Og in 2002 and 2005; it has a half-life of 0.7 milliseconds.

The below table contains various combinations of targets and projectiles that could be used to form compound nuclei with Z=118.^{[citation needed]}

In 1999, a team led by Victor Ninov at the Lawrence Berkeley National Laboratory performed this experiment, as a 1998 calculation by Robert Smolańczuk suggested a promising outcome. After eleven days of irradiation, three events of ²⁹³Og and its alpha decay products were reported in this reaction; this was the first reported discovery of element 118 and then-unknown element 116.^[5]

The following year, they published a retraction after researchers at other laboratories were unable to duplicate the results and the Berkeley lab could not duplicate them either.^[6] In June 2002, the director of the lab announced that the original claim of the discovery of these two elements had been based on data fabricated by principal author Victor Ninov.^[7][8] Newer experimental results and theoretical predictions have confirmed the exponential decrease in cross-sections with lead and bismuth targets as the atomic number of the resulting nuclide increases.^[9]

Following successful experiments utilizing calcium-48 projectiles and actinide targets to generate elements 114 and 116,^[10] the search for element 118 was first performed at the Joint Institute for Nuclear Research (JINR) in 2002. One or two atoms of ²⁹⁴Og were produced in the 2002 experiment, and two more atoms were produced in a 2005 confirmation run. The discovery of element 118 was announced in 2006.^[2]

Because of the very small fusion reaction probability (the fusion cross section is roughly 0.3–0.6 pb), the experiment took four months and involved a beam dose of 2.5×10¹⁹ calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of oganesson.^[11] Nevertheless, researchers were highly confident that the results were not a false positive; the chance that they were random events was estimated to be less than one part in 100,000.^[12]

In a 2012 experiment aimed at the confirmation of tennessine, one alpha decay chain was attributed to ²⁹⁴Og. This synthesis event resulted from the population of ²⁴⁹Cf in the target as the decay product of the ²⁴⁹Bk target (half-life 330 days); the cross section and decays were consistent with previously reported observations of ²⁹⁴Og.^[10]

From 1 October 2015 until 6 April 2016, the team at the JINR conducted a search for new isotopes of oganesson using a ⁴⁸Ca beam and a target comprising a mixture of ²⁴⁹Cf (50.7%), ²⁵⁰Cf (12.9%), and ²⁵¹Cf (36.4%). The experiment was performed at 252 MeV and 258 MeV beam energies. One event of ²⁹⁴Og was found at the lower beam energy, while no decays of oganesson isotopes were found at the higher beam energy; a cross section of 0.9 pb for the ²⁴⁹Cf(⁴⁸Ca,3n) reaction was estimated.^[13]

In the 2015–2016 experiment, these reactions were performed in a search for ²⁹⁵Og and ²⁹⁶Og. No events attributable to a reaction with the ²⁵⁰Cf or ²⁵¹Cf portions of the target were found. A repeat of this experiment was planned for 2017–2018.^[13]

This reaction was originally planned to be tested at the JINR and RIKEN in 2017–2018, as it uses the same ⁵⁰Ti projectile as planned experiments leading to elements 119 and 120.^[14] A search at RIKEN using this reaction (with the 3n, 4n, and 5n channels leading respectively to ²⁹⁵Og, ²⁹⁴Og, and ²⁹³Og) was unsuccessful.^[15][16] The experiment ran for 39 days in 2017, before it was paused to search for element 119 in the ²⁴⁸Cm(⁵¹V,xn)^299−x119 reaction instead. An upper limit of 0.50 pb for the cross-section was obtained; this is the same cross-section for the successful ²⁴⁹Cf(⁴⁸Ca,3n)²⁹⁴Og reaction (0.5+1.6
−0.3 pb) and an order of magnitude greater than the theoretical cross-section for the ⁵⁰Ti reaction (50 fb). This is consistent with the experimental cross-sections of ⁴⁸Ca- and ⁵⁰Ti-induced reactions yielding livermorium isotopes. The RIKEN team estimates that the necessary sensitivity level for the production of oganesson isotopes in the ²⁴⁸Cm+⁵⁰Ti reaction could be reached with 50 days of irradiation at a 1 pµA mean intensity, which is realistically achievable given the technological possibilities available at experimental facilities in 2025.^[17]

Theoretical calculations done on the synthetic pathways for, and the half-life of, other isotopes have shown that some could be slightly more stable than the synthesized isotope ²⁹⁴Og, most likely ²⁹³Og, ²⁹⁵Og, ²⁹⁶Og, ²⁹⁷Og, ²⁹⁸Og, ³⁰⁰Og and ³⁰²Og.^[18][19][20] Of these, ²⁹⁷Og might provide the best chances for obtaining longer-lived nuclei,^[18][20] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around ³¹³Og, could also provide longer-lived nuclei.^[21]

The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.

DNS = Di-nuclear system; 2S = Two-step; σ = cross section