It's not the same. The rule is when it is found with ou or outou it means until the time when. See again Thayer's Lexicon http://www.blueletterbible.org/cgi-bin/words.pl?book=Mat&chapter=27&verse=8&strongs=2193&page=1&flag_full=1
This rule for use of EWS does not apply in Matthew 27:8
"ou" in Mt 1:25 simply points to the time, "till such time as".
Matthew 26:36 also has "eos ou", but there is no indication the disciples were to leave as soon as Jesus was done praying. See also 2 Peter 1:19, where "until" would be an innatural reading.