Yes, they are. Nearly all spiders have venom glands, but Orb-weaver bite is not dangerous to human life (except for very rare cases).
The following text would convince you that we should not be afraid of Orb-weavers, but we also should not trivialize dangerousness of them:
First, it should be noted, in which cases Orb-weaver can bite you. Orb-weavers bite only in self-defense, when they are squeezed or are otherwise limited in their movement. Orb-weaver never jumps from its web on human to bite him. Orb-weavers are timid. When you accidentally get tangled in Orb-weaver web it will run away.
Orb-weaver can also bite you only by improper handling with it or when you accidentaly squeeze the spider (e. g. in its nest). Small Orb-weaver species don´t have enough strong mouthparts to bite through human skin. Bigger species can do it, but their venom is usually not toxic to humans. Let's look into the scientific literature on symptoms caused by Orb-weaver bite:
The primary symptoms are local - pain, slight swelling and redness. Nathanael McKeown from Oregon presents in his study from 2014 symptoms caused by bite of two big Orb-weaver species: After the bite of Diadem spider (Araneus diadematus), symptoms such as redness, pain, itching and swelling lasted in range of 1-17 days. In this condition systemic symptoms such as anxiety, nausea, headache and muscle cramps were sometimes reported. In one case, the Fierce Orb-weaver (Araneus saevus) bite caused more serious symptoms like pain, swelling, fever and numbness. These problems persisted 12 weeks. Fierce Orb-weavers are thankfully quite rare, so getting bitten is not so common situation.
Stronger reaction to Orb-weaver bite may be common in kids and seriously ill people. Orb-weaver venom may also cause an anaphylaxis (type 1 allergic reaction). It is a serious reaction that is rapid in onset and may cause death. It can manifest as shortness of breath caused by spasms of the bronchial muscles or as anaphylactic shock where the heart can failure by the severe decrease in blood pressure. Anaphylaxis occurs as the response to a venom substance against which IgE antibodies were made during the immune response to an earlier exposure. After the first contact with the substance the white blood cells produces IgE-antibodies. During the second contact with the same substance, IgE antibodies bind to it. After that many inflammatory mediators are released into the circulation and they cause symptoms of allergic reaction (e. g. rash, itching, rhinitis, cough, shortness of breath, nausea, diarrhea, anxiety, headache, low blood pressure, heart rhythm disorders). Lighter forms of anaphylaxis may have only mild symptoms, severe forms may end up with anaphylactic shock and death. There are two essential conditions for the development of allergy: 1. Genetic predisposition 2. Repeated bites. If you don´t take Orb-weavers into your hand you should not be afraid of them. Bees, wasps and hornets are clearly more dangerous then Orb-weavers because they often stab the same person repeatedly. Severe anaphylactic reaction to Orb-weaver poison is only theoretically possible, there is probably only one case report in the world:
In 1976, Zvonimir Maretić and Olga Milina reported a case of Walnut Orb-weaver (Nuctenea umbratica) bite on arm of a woman. The symptomatology was characterized by local pain, oppression on the chest and a sense of choking. Muscle cramps, weakness and paresthesias spread in all four limbs. Patient also felt pains in lumbar and abdominal regions. She complained of generalized itching, sweating, nausea, and excessive production of saliva. Burning of the soles of her feet, muscle pain, insomnia, and paresthesias lasted several days. These symptoms are similar to latrodectism. Latrodectism is the illness caused by the bite of the black widow spiders and related species (Latrodectus spp.). This syndrome includes symptoms such as pain, muscle cramps, sweating, fever, nausea and diarrhea. The patient can die from renal, respiratory and circulation failure.
As already mentioned, this is probably the only case report where Orb-weaver bite caused severe symptoms. It was definitely not a normal reaction. This text doesn't compel you to be afraid of Orb-weavers. From a medical point of view, it is desirable to provide information truthfully and completely. As regards to the above-mentioned study from Oregon, there was a very small number of bites, so we can not make any conclusions. Orb-weaver bite is very rare and mostly undesirable so we don´t have any credible medical studies. It always matters on the place of the bite, on the amount of poison and on health, physical and mental state of human. In several cases, even we (authors of this website) have been bitten by Orb-weaver (Araneus genus) by improper handling with them. Pain, swelling and redness mostly stopped the next day.
In terms of first-aid for spider bite find the appropriate recommendations for your country.
Míková K., Jedličková H., Řezáč M. & Macík S. (2015): Kousnutí pavoukem v ČR i ve světě [Spider bite in Czechia and in the World]. Čes. Dermatovenerol. 5 (2): 107–112.
McKeown, N., R. S. Vetter, and R. G. Hendrickson. (2014): Verified spider bites in Oregon (USA) with the intent to assess hobo spider venom toxicity. Toxicon 84, 51–55.
Herle P. (2009): Bakteriální choroby kůže v ordinaci praktického lékaře. Lékařské listy, 2009, 19.
KAZBUNDOVÁ J., PATOČKA J. (2006): Jedovatí pavouci a nebezpečí jejich kousnutí. Vojenské zdravotnické listy, 75 (2): 65-68.
Maretić, Z., Milina, O. (1976): A bite by the spider Aranea sexpunctata Linné: case report. Toxicon 14, 392-393.