I keep it short & sweet:

Can you enter any EU countries with a one-way ticket?
(reasons being don't yet know departure date or whatever)

Will you get hassled in Italy and/or Spain with a one-way ticket?

How about other countries?

Let me know what you know.

This depends on the visa requirements. If you don't have a return ticket, other requirements may become more stringent (e.g. funds your bring along). If you are a US citizen, you don't need a visum but only a valid passport.
Once you obtained a Schengen visa, you can travel freely through all Schengen countries

Chinani

One way Tickets usually will raise red flags, except you are a US or EU citizen. Buy a round trip Ticket and purchase Insurance that will let you get a full refund. Read the fine prints very well. www.priceline.com or www.hotels.com. I like using those sites when i visit Europe.

Good Luck.

My Aunt and Cousin who both hold American passports had to get return tickets when they were visiting EU last year(Italy, UK, Ireland), first destination for them was Ireland.
I think if you are coming as a visitor you may have to have return tickets, except you are on a working holiday visa or similar. You should check and confirm the entry requirements for the first country you are visiting/arriving at.