This preparation is quite similar to Sustanon, and is different only in that 100 mg/mL of it (of 250 mg/mL total) is testosterone hexanoate instead of the testosterone decanoate used in Sustanon. For this reason, Omnadren has a shorter half life, and will give a faster initial increase in blood level. This accounts for the claim of increased water retention and increased side effects, since levels, at first, are higher for the same dosage.
The claim that Omnadren has a duration effect of "a good 2-3 weeks" is somewhat misleading since the half life of the longest lived component is only about 5 days. There is of course some effect 2 or 3 weeks after injection, but relatively little.
The hexanoate ester is quite similar to the well known enanthate ester, but is shorter by one carbon.
The isohexanoate ester in Omnadren is the same, only named differently, as the isocaproate ester in Sustanon. Thus, the hexanoate vs. decanoate difference is the only difference in the mixture of esters.