In order to translate/localize street-fields labels:

• explode street fields from Stores > Configuration > Sales > OneStepCheckout for either billing or shipping fields 
• open up i18n/en_US.cs and find the Street* fields . It is suggested to maintain such changes in your theme scope and in your own locale files. 
• Translate the fields as follows:

"Street Address","Street Address",module,Onestepcheckout_Iosc
"Street Address 1","Street Address 1",module,Onestepcheckout_Iosc
"Street Address 2","Street Address 2",module,Onestepcheckout_Iosc
"Street Address 3","Street Address 3",module,Onestepcheckout_Iosc

can be localized as follows:

"Street Address","Straat",module,Onestepcheckout_Iosc
"Straat 1","Huisnummer",module,Onestepcheckout_Iosc
"Straat 2","Toevoeging",module,Onestepcheckout_Iosc
"Straat 3","Street Address 3",module,Onestepcheckout_Iosc

Trick is that Magento2 uses double localization for street-field labels on frontend and thus you need to localize any following line by your first localization. First localized value is not carried over from backend localization and is reassigned for localization on frontend based on first value. To overcome this just localize following fields in localized form.