During my training as PADI-divemaster I have learned that there is a risk for an reverse squeeze or an reverse block when diving during a cold or allergy combined with the use of a nasal decongestant. A medical doctor in another diving club (VDTL-club) stated that there was no risk at all when using a long-working decongestant like xylometazolinehydrochloride (Otrivin). He stated that you should use the spray half an hour before your diving activity. I think this is a rather confusing situation and I look forward for your answer.

The use of very short acting decongestants, in the presence of an acute URT (Upper Respiratory Tract) infection / inflammation, may, teoretically generate a situation by which the effect of the drugs fades out during the dive and puts the diver at risk of a reverse block on ascent. Long acting preparations ( those for which the indications suggest use every 8 - 12 hours or so) should, theroretically expose to reduced risk of similar occurrences. The recommendation of the second Diving Doctor should also be interpreted in that sense, i.e.: using the decongestant shortly before the dive should assure both enough time for the drug to act an protect during descent and to be still active at the time of ascending from the dive.
The two statements are non conflictual, but actually represent well the two sides of the same coin, as they both refer to the effects and side effects of this kind of drugs and to how to correctly manage them. Whatever the choice of drug and assumption timing, if the clinical condition is that of an acute URT inflammation / infection, the risk of barotrauma is increased, be it during descent or ascent.
An advice that the two statements both miss to provide is the "common sense and good diving practice" one: "if you are in a situation which requires the use of such a drugs, your may well consider to skip the dive for that day!