If the battery is specified for use in an Olympus E330, then you should be ok, it should work. The caveat here is that your camera manual will state that only the correct battery (i.e. theirs) should be used. Of course, this is a reasonable statement to make, they will have control over their own batteries specifications and manufacturing parameters. They will know what the tolerances on the camera are as well.
I've used replacement batteries before with my Canon 350D, the replacement battery was from Hahnel. Worked fine, no problems. That said, you hear lots of stories of problems with replacement batteries as well. I also prefer to use original batteries whenever possible. I consider the Hahnel brand an acceptable alternative to Canon's own.
The small differences in voltage between different packs is due to the chemistry of the cells in them. Depending on the chemistry used, many Li-ion batteries typically have a potential of 3.6V / 3.7V (there are other Li-ion chemistries and associated voltages). In order to achieve the voltages required by cameras these are typically stacked as a pair, doubling the voltage to 7.2V or 7.4V. Spot the connection with your two batteries? Too low a voltage means the device cannot operate, too high could indeed damage the camera, but there is a reasonable window of safe operation around the 'required' voltage. I'd like to think that Olympus engineers have designed the device to be able to cope with a 0.2V difference.
I'm going to cover myself by saying if you have any doubts don't use the replacement. There are other factors to consider with cheap replacement batteries other than the voltage such as the quality of the design and safety. How is the cost lowered so much? Labour costs? Battery design? Operating lifetime? Safety measures cut out? Maybe someone has got hold of a load of overstock cheaply and the batteries really are a bargain. Problem is, you never know. The use of a non-specified replacement battery is also likely to invalidate your warranty.
You did the right thing by asking others of their experiences with this battery in this camera. I suggest you nip over to DPReview and have a search of their forums.
Erm, is this thing on?