There is a Zulip discussion thread on how to order medical gases in FHIR (https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Example.20of.20oxygen.20prescription).

In response to this thread some examples have been posted on using MedicationRequest to order medical gases.
The examples (HeliOx therapy) are given below:

HeliOx (helium/Oxygen therapy) order
Gas = HeliOx 70/30%
Flow rate = 6 L/min (can be 6-8 L/min)
Device = non-rebreather mask | CPAP mask | biPAP mask | endotracheal IPPV
Clinical indication examples: SCTID:71186008 Croup (disorder) | SCTID:13645005 Chronic Obstructive Airway Disease (disorder) | SCTID:195951007 Acute exacerbation of chronic obstructive airway disease (disorder) | SCTID:708090002 Acute exacerbation of asthma (disorder)
This is how I consider the order data can be represented in FHIR

MedicationRequest.medicine.code = SCTID: 89569400 - helium 0.79 milliliter/1 milliliter and oxygen 0.21 milliliter/1 millilite (apology - can't find the HeliOx 70/30% SCTID)
MedicationRequest.dosageInstruction.dosage.doseRate[x].rateQuantity = 6 L/min (or .rateRange = 6-8L/min)
MedicationRequest.device.DeviceDefinition.deviceName.name = non-rebreather mask | CPAP mask | biPAP mask
MedicationRequest.device.DeviceDefinition.deviceName..type = user-friendly-name
MedicationRequest.reason (codeableReference - Condition) Condition.code = SCTID:71186008 Croup (disorder) | SCTID:13645005 Chronic Obstructive Airway Disease (disorder) | SCTID:195951007 Acute exacerbation of chronic obstructive airway disease (disorder) | SCTID:708090002 Acute exacerbation of asthma (disorder)

HeliOx administration can be supplemented by O2 if patient's SpO2 <88%
Titrate supplementary O2 order example:
Gas = O2
Flow rate = 2 L/min (can be 1-3 L/min)
Precondition = if SpO2 < 88%
Device = nasal canula

The HeliOx and supplementary O2 order can be linked using:
MedicationRequest.groupIdentifier

There is a suggestion on this Zulip discussion thread to add the examples to the MedicationRequest resource
This JIRA request is to add the examples as suggested