Nasal decongestants such as oxymetazoline (Afrin and generic) can temporarily reduce the swelling of nasal mucosa and may facilitate middle ear and sinus equalization during diving. Some degree of 'rebound' (increased mucosal swelling) can occur after the effects of the drug wear off. However, the effects of oxymetazoline usually last for up to 12 hours, thus it is unlikely that the effects will wear off during a dive unless the previous dose was administered several hours before the dive start. Some nasal decongestants (e.g. phenylephrine, Neosynephrine) have a shorter duration of action than oxymetazoline, and are associated with greater rebound. Prolonged use of any nasal decongestant can be associated with severe rebound nasal swelling; prolonged use is discouraged beyond what is necessary to provide short term relief of symptoms or to promote middle ear or sinus pressure equilibration. For oxymetazoline nasal spray, once daily use for up to 5 days is unlikely to be associated with significant rebound effect after it is stopped.
It is interesting to follow the developments with regard to nasal decongestant use and diving. There seems to be a change in the way that decongestants are viewed from a diving perspective. The use of decongestants has been a source of many debates in the diving community. The one camp (mostly dive training organizations etc.) laid down strict guidelines that decongestants should not be used before diving. The other group felt that there was a place for decongestant use, especially long acting ones.
The main issue appears to be the problem of reverse squeezes. In a reverse squeeze situation, the effect of the decongestant wears off during the dive and upon ascent, the pressure in the inner cannot equalize to the ambient pressure of the water around the diver (in the ear canal - outer ear). This is a very painful situation that can lead to serious medical problems. The real problem is that a diver could be faced with making a decision of either rupturing an ear drum or running out of air. Not a good choice.
Disclaimer: please consult your physician about taking any medication before diving - especially decongestants. The safe and prudent thing to do is of course to consult a physician. If you can find a physician who is a diver or is experienced with diving medicine all the better.