The Old Catholic Church is a schismatic group that separated from the Roman Catholic Chuch for various reasons over the years. It is most clearly identified with the separation over the doctrine of Papal Infallibility. You can get some history at https://en.wikipedia.org/wiki/Old_Catholic_Church
To be Catholic in full communion with the Church, a person must be joined through the profession of the same Faith, Sacraments, and ecclesiastical governance. To be Catholic you must have all 3.
Every schismatic group has voluntarily separated itself from apostolic succession once they leave the Roman Catholic Church. The Old Catholics do not recognize anyone above themselves and have not recognized the ecclesiastical authority of the Church since the mid 1800s. Strike 1.
Inevitably schismatics stray from sound teachings once they have lost the important tie to the Deposit of Faith. As we have noted, they deny the Doctrine of Papal Infallibility and do not recognize any other established Church teachings after the 7th Ecumenical Council. Strike 2.
The Old Catholics give the appearance of sharing the 7 Sacraments, but this is highly questionable when examining their orders. They have close ties to and accept protestant denominations. This has ultimately led them to ordaining those incapable of receiving orders such as apostates and heretics from other denominations including women, homosexuals, and married clergy. While many recognize orders of the Old Catholics, others strongly question the validity on grounds that besides being illicit, there is intent to circumvent the authority of the Church and create an unregulated parallel heirarchy.
Additionally, the outright denial of established Catholic doctrine causes the heretic to automatically lose office and all faculties are suspended. They have lost the ability to confer orders. Finally review the next section regarding Cannonical Status which cements the argument that being excommunicated, their orders cannot be valid. Strike 3!
Rather than reinvent the wheel, let the Vatican explain the situation:
"While the many formerly Roman Catholic clergy (priests and deacons) in the Old Catholic Church are of great significance for the pastoral ministry of these churches, they do impede the path to ecclesial communion because of the residual jurisdictional bond with the Roman Catholic Church (can. 11 CIC). The sanctions to which they continue to be subject in the Roman Catholic Church are excommunication for heresy and schism (can. 1364 CIC) and suspension in the case of marriage (can. 1394 § 1 CIC). Both censures deny the priest the exercise of his ordination and his jurisdictional powers (can. 1331 § 1, 1333 § 1 CIC). On the basis of suspension because of marriage they are by law relieved of their ecclesial office in the Roman Catholic Church (can. 194 § 1 no. 3 CIC)." Please see "The Church and Ecclesial Communion Report of the International Roman Catholic-Old Catholic Dialogue Commission at the end of this page. See section 220.127.116.11 Full PDF available.
The original link below has been corrupted and the document is no longer available through the Vatican archives. PDF below is a retrieved archival copy of the document in it's entirety.
Without further comment, you can see it is best to avoid these people and any poisoned fruit they have produced. They are NOT Catholic despite what their name claims.