Originally Posted by
pinniped
That's dependent on the country, your own citizenship, and the type of visa you hold.
When I had my open-ended ticket, I was entering the UK with a US passport and permission to stay 1 year. I don't recall how closely immigration looked at the airline ticket, but I know I handed the agent a healthy stack of paperwork and he looked through some of it. This was in an era when you couldn't easily buy one-ways to begin with, so I'm guessing the agent was used to that type of ticket. Given the ubiquity of one-ways today (at least on on a US-UK route where you could easily show up on Norwegian or something), I don't know whether immigration agents view the fact that you haven't yet bought your return as any kind of concern.
Whenever I enter Europe or the UK without a visa - either short-term business or as a tourist - nobody ever looks at my airline itinerary. Same when I've entered other countries with business visas. Only place I had to show all my paperwork was Bermuda of all places...and I was just going there for the weekend as a tourist. (I had it printed on paper, like I always do...just in case.)
There is a common perception around the issue of 'proof of onward travel'. Many people believe this is always needed when in fact it may not be. Some countries do require you to show proof of onward travel (or return) and some do not. Airlines are far more likely to be who insists on it as they are subject to penalties and having to fly you back to your departure point if you are denied entry into a country.
If you ask an airline if you need a return ticket to the UK for example, the airline may well say, 'yes you do'. Yet there is no requirement for a return ticket in UK Immigration law they can point to as it doesn't exist. What many countries such as Canada (as another example) do have is a requirement, 'to satisfy the Immigration Officer of your intent to leave', or words to that effect. A return or onward ticket can provide some indication of that intent but still isn't proof of intent. The bottom line is if the Immigration Officer 'suspects' your intent, that's all that is required to deny you entry whether you have a return ticket or not. All the return ticket does is cover the airline's butt, not yours.