Don't be afraid of opening your mouth. It doesn't matter if some wrong words are jumping out. They understand your situation and really don't care about your mistakes.
Even sometime you can find out their grammetical mistakes too.
Oral is just oral. Say more, learn more.
Try to have lunch at the same table with your colleagues. Talk to them bravely.
Don't be shy asking questions if you don't understand.
Pay attention to what they are chatting and remember what the words or phrases they like to use, then use them if you feel it is applicable.
The basic is:
SPEAK OUT.