Is it ok for a boy to wear girls underwear?

No its is quite wiered idea and is also not a sign of normality and if you are going to wear a used underwear then it also may cause some of the allergic problems

Actually more common than you think however if you are going to wear someone elses you should have the courtesy to let them be made aware of it

It's just fine. Just make sure you/they wash "yourself/themselves". I myself am going to start wearing my girlfriend's.

Ignore the naysayers, wear whatever you prefer. Many people will be judgmental because they are not mature enough to 'not care' and 'mind their own business'. Whatever your comfortable wearing is good for you, but as others said, wear clean ones or purchase your own.