There is debate about how to keep the Or Zarua's onah. For example, many poskim who hold by onat Or Zarua do not expect a woman to observe onat Or Zarua for veset hachodesh or veset haflagah that are not kavua.
Our understanding of common practice is:
1) To observe the onah preceding the veset hachodesh, day or night.
2) To observe the onah preceding the onah beinonit. If she last became niddah at night, this would be the daytime before. If she last became niddah during the day, this would be the night before.
Some couples observe a full 24 hours (night followed by day) for onah beinonit. For such couples, if the woman last became niddah during the day, the onat Or Zarua would overlap with the night onah she is keeping as part of the 24 hours.
Some couples keep both the 30th and 31st days as onah beinonit. Such couples observe the onat Or Zarua only for the 30th day.
3) To observe the onah preceding the veset haflagah, day or night.
4) A woman does immerse and have relations when her onat Or Zarua falls on mikveh night. In this case, she should perform a bedikah prior to relations.