What is Ultherapy aftercare? Is it fine to have oxygen facial few days after my Ultherapy procedure?

nothing interferes with Ultherapy I say combine fractora/morpheus8 and botox to help give you more improvement.As we age the skin sags there is  nothing  you can do that but try preventative  energy-based or thread-based  treatments or get a neck/facelift earlier in the aging process.  There are numerous options and some can be used in combination to get best outcomes but you need a consultation with an expert.
Options include for skin tightening :
thermage radiofrequency to heat skin and tighten it over time and prevent the aging process
ultherapy highintensity ultrasound to contract muscle and tighten the face and neck and prevent the aging process
PDO nova threads or instalift threads for immediate lifting without surgery and long term collagen production
sculptra injections for collagen production to areas of the face and neck with crepe skin or early sagging
btl exilis ultra, venus legacy, thermi250 , or inmode forma/contoura  for constant collagen stimulation monthly with facials
facelift/necklift with fat grafting/fillers if you need a big improvement
ThermiRF or J plasma /Renuvion or Inmode FaceTite   internal heating and tightening to long term internal heating and tighting with or without surgery
Platysmaplasty to tighten the muscles of the neck long term and create contour
Facial implants or fillers to contour the jawline and chin to improve the apperance of the nec k
Fractora/Morphenus8 or Intensif microneedling RF for improve superficial tightening, neck and chest wrinkling and crepe skin 
You need to see an expert in facial sculpting and age prevention.  Best, Dr. Emer.

You would not have to hold off on facials and skin care after Ultherapy.  You would want to avoid neurotoxins (botox or xeomin) and  fillers for 2 weeks after Ultherapy.  But facials are fine to do right away.  The Ultherapy treatments lays tiny blasts of heat and energy deep under the skin.  Once they are there, there's no "undoing" what as been done.  Doing a facial on the surface of the skin will not harm the neo-collagenesis occurring deep under the skin from the Ultherapy treatment.