I believe (I'm a beginner in Welsh, however) that it's because the number is specified. If you a referring to a non-specific quantity of oranges, then 'the oranges' would be 'yr orennau'. However, with the addition of a number, Welsh has an unusual quirk whereby the singular form is used - pymtheg oren, for example.
Yep, as you said. After a number in Welsh we use the singular form of a noun, not the plural.
As in "two orange ones"? Yes, you could say "Ges i ddau oren" (I got two oranges / two orange ones).
No I meant an orange number two, e.g. in a drawing. (Or dau glas, dau coch etc. ). Maybe a silly question, but I just wondered :-)
Haha, I did wonder. As in a Sesame Street kind of thing? Sure, yeah, you're right. Watch some Welsh kids' programmes and you'll probably come across it!