How do I interpret odds ratios in this scenario? I measured the odds ratio of transaction by comparing the usage of one marketing channel as opposed to not using that channel. My odds ratio is 2.56. Now I found the odds of having transaction  is 
about 2.56 times greater for those who use this marketing channel  than those who do not use this channel. But I have difficulty to interpret this number. Does it mean  1 point change in the visitors from this marketing channel  results in increasing 256% in transaction? But it does not make any sense. One visit at most can result in one transaction. 
Can I use logit regression and say $\ln 2.56 = 0.94$ so one point increase in the channel increase 0.94 of transaction?
 A: I'm not sure if I understand your question correctly, but I think it all depends on how many visitors come from  the marketing channel, vs how many visitors come from other sources, and on how likely is a visitor coming from other sources, to make a transaction in the first place. 
Denote with $N$ the total number of visitors, with $N_A$  the number of visitors from the marketing channel and with $N_B=N-N_A$ the number of other visitors. Suppose also that the probability of a transaction for a visitor from another source is $p_B$. Then the probability of a transaction for a visitor from the marketing channel is $2.56p_B$. The expected number of transactions is thus
$$\mu=p_B(2.56N_A+N_B)$$ 
All other things being equal, one more visitor from the marketing channel leads to an expected number of transactions equal to
$$\mu^*=p_B(2.56N_A+N_B+2.56)$$
Thus the fractional increase in the number of transactions is 
$$\frac{\mu^*-\mu}{\mu}=2.56\frac{p_B}{\mu}$$
As noted above, this formula is only valid if the only change is the unit increase in the number of visitors coming from the marketing channel. The number of visitors coming from other sources, the probability of transaction for the visitors from other sources and the odd ratio are all assumed to be constant: the only change is in $N_A$.
A: You have already given the right interpretation, as far as I can tell:

Now I found the odds of having transaction is about 2.56 times
  greater for those who use this marketing channel than those who do not
  use this channel

Your independent variable is (apparently) using the channel vs. not - and it takes values 0 or 1.  If you want to say something about another independent variable (such as number of visitors) then you need to run a different regression. 
A: Odds are a ratio of probabilities and an odds ratio, naturally enough, is the ratio of the odds.
In this case, your odds are $\frac{P(Transaction ~|~ Marketing)}{P(No ~Transaction ~|~ Marketing)}$ and similarly for no marketing. The odds ratio is telling you that given your marketing channel, the odds of someone making a purchase are 2.56 times greater than if you don't use it.
