If the wavelength of Hydrogen is 21.10611405413 cm,
what is the wavelength of oxygen in cm?
It ain't that easy to extrapolate from single-electron atomic spectra to the spectra of polyatomics and molecules. Applicatuion of quantum mechanics becomes exceedingly messy.
The 21 cm line of hydrogen is due to a spin flip of its single electron. Oxygen has a much more complex microwave spectrum, with more than one line, unless you are talking about the O(7+) ion. There is not much of that around in outer space. In astrophysics, the main application of oxygen's microwave spectrum involves planetary atmospheres, where O2 is the species doing the absorbing.
The microwave spectrum of O2 is shown and discussed at
http://www.radiometrics.com/Strandberg_PhysRev49.pdf
To find the wavelength of oxygen, we first need to know the energy level transition or the spectral line for oxygen. Hydrogen has several series of spectral lines, such as the Balmer series, Lyman series, and Paschen series. However, oxygen's spectral lines are not as well-known or commonly used in physics and chemistry.
If you have a specific oxygen spectral line or energy level transition in mind, please provide that information, and I can assist you further in calculating the corresponding wavelength.