My oxygen dependant ex prem baby starts with snotty nose which then totally blocks, so that the oxygen can't get throygh but its not visible?

Saline and suction. Saline nose drops and suctioning help to clear the mucus; you can do this whenever needed. If you do use a humidifier, it needs to be scrubbed out thoroughly daily so it won't put mold and mildew into the air. Dr. Ferguson's suggestion about trimming the nasal prongs slightly is a good one, too.
Try gental cleaning. The tiny nasal passages of infants often fill with mucoud debris and a constant effort too clear them is necessary. Sometimes the nasal prongs will irritate so triming them may help.Using lots of saline to wash out or thin the mucous helps.Avoiding smokers/perfume/candles/incense or other irritants is a must as is a humidifier if you have dry heat in the winter.