"OTM" means "other than Mexican", and refers to Central Americans, South Americans, Middle Easterners, North Koreans, etc. who are caught sneaking over our southern border. The article "Officials: OTMs 'a very grave problem'" states that many crossers choose Texas as the place to cross because they know they won't be detained due to limited detention space. Not only that, but:

"Down in Eagle Pass, we'll see them sitting by the side of a road, and they'll chase us down and nearly beg to be arrested," said one field agent who asked to not be identified.

"They know they’ll be released pretty fast, and that paper (court "notice to appear" document) is like their ticket to go wherever they want."

...Berg explained that OTM release actions have been widely broadcast throughout Central America and beyond, causing the Texas section of the U.S. border, from Del Rio to Brownsville, to be the geopolitical objective in the sights of millions of would-be border crossers...

From October 2004 to the present they detained 42,239 illegal aliens. 14,569 of which were OTMs, and about 13,000 of those were released into the U.S.

See also:
"Illegals detained at border released onto U.S. streets"
"Bonilla demands review of OTM release policy"
"Illegals from terrorist nations are crossing the border into Arizona."