O-Ring Gag

I'm looking to try an O-Ring gag for the first time, I think my boyfriend would love the idea of me being able to perform oral while wearing a gag. A lot of the products I've looked at have reviews saying that the O-ring isn't big enough for this though, so I'm wondering if anyone can recommend a product which has worked for them :)

Also, just any advice about O-ring gags would be nice - how does it feel to wear one, if I use one while performing oral will my teeth get in the way, etc

If any of them are going to be big enough, it's this one: http://www.lovehoney.co.uk/product.cfm?p=15737 A diameter of 1.75" is a circumference of 5.5". Knock off half an inch to be able to move and really it is only going to be big enough if your partner has a circumference of less than 5". The average erect penis is 4.59" when erect (stats from: http://www.nhs.uk/Livewell/penis-health/Pages/penis-size.aspx ) so lots of men would be able to fit in this O ring but it really depends on your partner's circumference.

I've given blow jobs through an O ring gag (not sure what brand so can't recommend). It felt a bit odd and it was strange not really being able to do much with my mouth to make it more pleasurable for him. it made me feel particularly vulnerable and submissive, which I liked. No teeth issue for me.