Servicing the oxygen in most cases requires a tool. Any maintenance that is done must also be done in accordance with company and manufacturer maintenance documents. Oxygen service has a number of applicable particulars, from the type of tool (does it produce a spark, is it free of grease, will it round off fittings, what is the minimum or maximum value for seating the valve, how does one determine valve leakage after shut, etc. Oxygen service values depend on the temperature of the tank, and if one is servicing to a particular value, it must be determined based on the temperature of the tank and ambient temperature; one can't simply say the tank takes 1,800 psi, when on a cold day full may be much less. One has to. know what the maintenance manual requires or stipulates, and the work must be done in accordance with that manual.
Any maintenance, including preventative maintenance, requires the act be settled via a log entry. If you use a system that involves a write up, closed with a log entry, then you require a write-up, to show that it was serviced.
Failure to make the required log entry places you in violation of 14 CFR 43.9. You're expected to do the work to the same standard, using the same practices, techniques, publications, tools, and standards as a certificated mechanic, and your paperwork must be up to the same standard, too. See 43.13.
When you fill that bottle, you become responsible for the work you've performed, and when you sign off that preventative maintenance and servicing, you sign for the work you just did, and buy the past: you absolve everyone else who serviced it, and put your name in the hot seat. Is that oxygen bottle within it's service life, or inspection interval? Did you check? If you serviced it, you just became responsible for it. Do you know where to find the hydrostatic test stamp, know the interval, know if there are outstanding airworthiness directives or other requirements applicable to that system?
If you break a line (open a fitting) on that oxygen system, you may introduce moisture or contaminants; there are flushing procedures for the system, depending on where you invaded it. If you twist a fitting which threads into another fitting, you require two wrenches to do that, and you need to know what you're doing to the other fitting. If yoiu apply any torque to the bottle or alter the tension on fittings to the bottle, you may cause a leak, which can be a flammability issue. Are you aware of the reaction of oxygen and grease?
If the oxygen system is low, at what value does that system require a full purge before it can be pressurized again, and is there a point at which the system must be tested, leak checked, or flow checked? Do you know? You shouldn't be touching that system if any of this isn't second nature to you, and if you don't know where to go to find out, or have access. In fact, you can't legally touch it. Additionally, unless you've been trained on it, and are legally allowed, then you can't. Does it involve complex assembly or disassembly? You can't do that, as preventative maintenance.
It should be apparent that servicing oxygen isn't just hooking up the hose and filling the tank. Not by a long shot. Yes, a maintenance entry should be made when oxygen is serviced. Unless you can legally make that entry and know what you're doing, then writing it up as a discrepancy, or needed service, then you should not make the entry, do the work, or sign it off.
The comment was made that writing a discrepancy is "grounding the airplane." This is the wrong way to view making a discrepancy report. If the airplane has a discrepancy, it has a discrepancy, period. Where is the operations manual authorization, operations specification authorization, or regulatory authorization to fly around with unwritten discrepancies to avoid "grounding the airplane?"
We've all heard the litany of excuses for not servicing oxygen. There's enough to get down from altitude. You're not going very far. We'll service it when it gets back tonight. The next station has equipment for that. Everyone has gone home. We'd have to pull the bottle. Yada, yada. I was recently in a foreign country and was told "we don't have a contract there, so we can't get oxygen," followed by, "there's more than enough to get to a lower altitude, if there's a depressurization." Really, and what if we have a cockpit full of smoke, mid-ocean? I haven't seen a maintenance or flight manual which cites justifications as to when the oxygen can be properly serviced and when we can get away with this or that. It's either correct or its not.
I flew for four different ambulance operations; urban, remote, dirt runways, international trips, you name it. Piston, turboprop, turbojet. I get it. Pilots performing preventative maintenance, from installing and removing the sled to servicing oxygen, etc, are typical at many operations, and there's at least one large operation that has long used welding oxygen as it's fill source (it actually does come from the same source...welding, medical, aviators breathing...despite what you might think). When something goes wrong with Baby Jane's isolette, or there's a problem with the system you last serviced, where do you think the bottle will point when it stops spinning, in court? To you. Most of us are aware that you an get away with a lot, until something goes wrong, and then the world takes a closer look. If you did the service, the music stops, you don't have a chair, you may find yourself in much deeper water than you care to tread.
Writing up a discrepancy carries some legal protection for you. Far more than flying around with an unwritten discrepancy. You identified the problem. You made it a matter of record. You noted something that must be addressed, and it became a matter of someone else shouldering the responsibility to make that happen. Conversely, flying about with maintenance discrepancies that are unwritten, means you're flying about in an unairworthy aircraft. It doesn't simply become unairworthy when you write it up. It's already unairworthy if it has a discrepancy that makes it so. If you engage in "shirt pocket maintenance," meaning you keep a list of items that need to be written up and then write them on the last leg of the day or the week as the case may be, then you carry a pocket full of liability, and nobody believes that it all coincidentally happened on the last leg.
It's a balancing act, but so far as oxygen service and maintenance, you're responsible for all the legal implications and requirements of that service, from the correct, current maintenance publication to the correct tools, to verification of any applicable airworthiness directives, recency of hydrostatic test, maintenance entries, etc, as spelled out in Part 43, and any other applicable documents (manufacturer instructions, flight or general operations manual, aircraft manual, maintenance manual, etc). This all falls on you, for as simple a thing as servicing that oxygen. The responsibility for it not being serviced, you carry, if you fly with it outside any limitations that apply to you, the aircraft, your operation, or your operator. How far are you willing to hang your neck out?