Hi,
Sorry for writing in English but I don't speak French

Osa is indeed an advanced AD asset. It is mobile and quite difficult to jam (though not impossible, as was indicated in the thread on acig.org forum). The main advantages of Osa are its mobility, the autonomy or each vehicle and the quality of the system (radar, missiles etc.). Two main drawbacks, however, are:
1) Its Maximum range (10 km) and altitude (5 km) mean that Osa can hardly engage targets that are using precision guided munitions (such as AGM-65D/G Maverick, or modernized Kh-29 missiles) so it is no longer so effective against air threats as it was when fighters used mostly dumb bombs, that's obvious.
2) Often forgotten, mentioned on acig.org forum thanks to which I've realised of it - its reaction time - at least 15 (or 26, according to different soruces) have to last from the target detection to the lanuch of the missile, which is little bit too long for short range AD unit (should be few seconds), and in turn it is little easier to avoid
Those drawbacks do not mean Osa is an obsolete system, even on very modern battlefield it can be effective against UAVs and helicopters, as the system is effective in tracking and engaging low-RCS low-speed targets (it has hit two UAVs during Lebanon War, for example, and during Victory Strike exercise in Poland Polish AD units equipped with Osa-AK had many "electronical hits" of AH-64A helicopters).
As far as the modernization is concerned, here goes the Polish one (serially produced, in service with Polish Army, more than dozen were delivered)
http://www.wzu.pl/English/Offer/Modernizacje/Zadlo/Zadlo.htmIn short: old electronical components are replaced with digital ones, an IFF is installed as well as the IIR camera - Sagem Matis, and the system is integrated into air defence command and control system. No modifications to the missile itself, but new missile might be installed on demand of the customer (it hasn't been done yet).
And finally, my question about the Osa, in fact I found the Forum looking for that information.
What is the Algierian data on the Morrocan aircraft shot down by SA-8?
I know, for example, that a Mirage F-1 was downed on 12 January 1985, but soruces are cotradictory, some saying it was Kub, other - Osa. Also, if you could, please inform me of any ground to air kills by Osa from Algerian Army you know.
Thanks and regards,
Jakub