From my experience, it's best to jump on the oxygen at the first sign of an approaching CH. The sooner the better. The longer you wait, the higher the pain level and that ends up making your oxygen abort times longer.
I'd also suggest you check out the anti-inflammatory regimen with 10,000 IU/day vitamin D3 or more along with the vitamin D3 cofactors. You can download a PDF copy of this treatment protocol at the following vitaminDwiki link:
I suspect the return of your CH is due to a drop in your 25(OH)D3 serum concentration (vitamin D3 concentration). Are you getting less direct sunlight than usual? We've found that a vitamin D3 insufficiency/deficiency is very common among CHers. The anti-inflammatory regimen can fix that deficiency and in the process, get you back CH pain free.
Take care and please keep us posted.