Psycho and ordinary numbers
Do not use namespace std:
There is multiple reviews on that here on this site already, so I'll just sum it up shortly:

using namespace std pollutes global namespace and leads to possible conflicts with other functions. For more information you can read this stackoverflow post on the matter.

Naming:
Simply because of your variable names I found it hard to understand what exactly you were doing. Characters are not costly. Anymore that is, but you should really write out what your variables are supposed to do:
od --&gt; oddFactorOccurence
ev --&gt; evenFactorOccurence
v --&gt; currentFactorCounter
n --&gt; number
t --&gt; countOfNumbers
hal --&gt; half / evalThreshold
erastho... no. just. NO. ;)