The death toll from deadly storms in the two states has risen to 21.
Forecasters have said more heavy rains are expected over the next few days, hitting the area with up to three more inches of rain.
This May has been the wettest month on record for Texas.
After a near dam break on Wednesday, rain-swollen rivers, creeks and lakes are now posing a threat to Texas and Oklahoma residents.
On Thursday, residents living in Wharton, a town southwest of Houston, were told to evacuate their homes due to a predicted rise of the Colorado River.
In North Texas, residents were asked to evacuate as well, because of the Brazos River reaching flood stage levels. People living in Western Oklahoma are evacuating homes, too.
Officials said at a press conference on Thursday that authorities are continuing to look for members of the families who were swept away in the flood in Wimberley, Texas.
Continued rain will hurt search efforts, and it is understandable to feel cynical, officials said, but efforts are not over.
Anywhere from 800 to 1,400 homes in Houston have been damaged by the storms.
President Barack Obama pledged federal assistance on Tuesday after Texas Governor Greg Abbott declared a state of disaster in at least 40 Texas counties, including Harris County, which includes Houston.
Elsewhere in Houston, 100,000 gallons of untreated wastewater spilled at a flooded wastewater treatment plant.
Meanwhile, Texas authorities defended the way they alerted residents to flooding dangers.
In Hays County, where a vacation home was swept away, authorities said they placed multiple alerts to mobile phones and calls to landlines.
National Weather Service warnings were received on people's mobile phones in Houston, but city officials said they do not have a system that lets them send more targeted warnings.
More rain is expected in the southern US as recovery efforts for missing people continue across Texas and Oklahoma.