Oasis Active is certainly one of the better free dating sites; it is a relatively new design, easy and fast to get started and quite simple to navigate.
Given the ease in which I created my profiles I would imagine it would not be difficult for a scammer to set-up theirs so one should keep their guard up as this is par for the course on free dating sites.
In oasis actives credit they have designed the site to minimize this risk as best as possible.
Generally people not paying for a service are not going to take it as serious.
In terms of the ads on oasis active I did find them somewhat distracting with banner ads on every page and some quite strategically placed below search results so they were quite easily confused with actual results.
I was comfortable in making the first approach and I did have mail and contacts coming in.