There is no universal regulation, ICAO Annex 6 does not mention portable oxygen equipment. However, both FAA and EASA have it in certification specifications for large aeroplanes, 14 CFR 25.1447 (c)(4) and CS 25.1447 (c)(4) respectively, and the wording is identical:
Portable oxygen equipment must be immediately available for each cabin crew member. The portable oxygen equipment must have the oxygen dispensing unit connected to the portable oxygen supply.
There is also a mention of portable oxygen in the air operations regulations, 14 CFR 121.333 (d) and CAT.IDE.A.235, respectively.
Note that neither authority specifically requires this equipment to be a bottle (cylinder).